A Once Human Sea Fortress is not a single item. It is what your ship becomes as you build and upgrade it in the Isles of Abyss scenario, which launched on October 21, 2026 (PT). You start with a skiff in Build Mode, then add hull sections, deck facilities, stronger propulsion and cannons as your Tech advances. Official materials call the top end a mobile base, workshop and heavy artillery platform, but they do not list one fixed recipe. Ship cannons matter most against Calamity Towers, Voyagers and the Sea Calamity Zone, and the Sunken Lord unlocks when your server’s purification progress reaches 100%.
Summary: Once Human Sea Fortress
- What it is: a fully upgraded ship that works as a base, workshop and artillery platform.
- Start: build a skiff in Build Mode, then expand it with hull, deck and facility parts.
- Upgrades: a parallel ocean tech tree takes you from sails to fuel and electric propellers and tiered cannons.
- Cannons: Harpoon, Long, Rapid-fire, Shotgun, Frost and Corrosion are available early, and Phase II adds more.
- Diving: shipwrecks and labs give treasure, while Skyfire Gold and Black Tide Fuel feed ship upgrades.
- Sunken Lord: opens at 100% purification progress, and the official notes describe a platform fight.
- Timeline: about 21 days in two phases, with a permanent server expected on November 12 (PT).
What Is a Mobile Sea Fortress in Once Human?
Starry Studio describes the ship progression as a ladder that runs from agile skiffs to massive Mobile Sea Fortresses. You craft a small vessel first, then customize and upgrade it. The studio says the ship can become a fully functional base, workshop and heavy artillery platform.
The developers treat ships as movable water territory. That means you place facilities on your ship the way you place them in a base on land. The official launch release adds that a parallel oceanic tech tree lets you upgrade from canvas sails to fuel and electric propellers, mount tiered cannons and harpoon turrets, and install automated fishing nets, smelters, generators and workbenches directly on your deck.
Because the studio calls the fortress the top end of the ladder, there is no single “Sea Fortress” blueprint to unlock. Instead, you grow the ship until it fits the description. For the launch details and release timing, see our Once Human Isles of Abyss release guide.
How to Upgrade a Skiff Into a Sea Fortress
The developers explain the build system in their October 16 dev blog. The steps below follow it, and they note that some values were still being adjusted before launch.
- Build your first ship in Build Mode. The first time, you construct a new water vessel from scratch. After that, you can use Relocation or the summon vehicle shortcut to place your existing ship.
- Assemble the hull. Dedicated components include the hull, gunwale, bow, deck, handrails and bulwark. Square, triangular, sloped and curved pieces let you shape the outline.
- Add smaller pieces. Small hull, deck and gunwale parts and pillars of different heights help you build the bow, stern and tight spaces.
- Choose a material. Wood, iron, steel and alloy options change the look, according to the dev blog.
- Save your design. The water platform layout has its own save entry, and you can select structures connected to the core deck and save them as a design.
- Upgrade the engine. Propulsion runs from manual power and sail power to fuel power and electric drive.
- Add handling gear. Boosters and side-thruster steering gear improve mobility.
- Mount cannons and facilities. Add cannons as your Tech unlocks them, then install workshop facilities on the deck.
As a result, your Sea Fortress grows with your Tech. The scenario ties higher gear tiers and ocean facilities to Tech progress, and characters start at max level. Therefore, the early game is about supply lines and a seaworthy ship, not leveling.
Boat Base: Workshop and Facilities on Deck
The studio confirms that you can install production buildings directly on the ship. Here is what the official materials list.
| Facility | What the sources say |
|---|---|
| Workbenches | Installed on the deck |
| Smelters | Installed on the deck |
| Generators | Installed on the deck, and Black Tide Fuel raises their Electricity cap |
| Trawl Net | Catches fish while you sail on the surface |
| Advanced Trawl | Increases the haul and salvages surface supplies |
| Tidal Cage | Breeds fish in the water, and the Advanced version has a shorter cycle |
| Drilling Well | Obtains abyssal sea oil |
| Repair facility | Continuously repairs damaged material and facilities once activated |
| Revive station | Automatically helps a nearby crawling character up |
The developers also optimized snapping rules so that hull pieces and facilities snap to fewer wrong positions. The build interface shows the category for your current territory first, along with a clearer range display. Nothing in the preview gives a fixed deck layout, so plan your own.
Ship Cannon Types and Placement
Cannons unlock as your Tech advances, and you switch between them with the Wheel while sailing. The studio notes that cannons with projectile drop get a pre-aim line. It does not publish exact mounting slots, so treat placement as your design choice.
| Cannon | Official role |
|---|---|
| Javelin Cannon | Fires tracking shells that hit small moving targets and weak spots |
| Harpoon Cannon | Loads quickly, penetrates the water surface and works against marine creatures |
| Long Cannon | Deals extra damage to decks and buildings |
| Rapid-fire Cannon | Shorter firing interval and faster shells, easier on the Calamity Tower’s weak spots |
| Repeating Long Cannon | Fires continuously for sustained output |
| Shotgun Turret | Covers a wide area, effective against enemy ships and patrol boats |
| Frost Cannon | Slows hit enemies so you can pursue them |
| Corrosion Cannon | Raises the damage enemies take, opening follow-up attacks |
Phase II adds four more: the Rocket Launcher for burst output, the Oil Drum Launcher for a big explosion that ignites the area, the Repeating Shotgun Turret for very high damage to Voyagers and the Freezing Cannon that freezes enemies. You can also choose the Ram for close-range impacts or the Naval Mine Launcher for traps.
A sensible loadout mixes roles. For example, pair a Harpoon Cannon for sea creatures with a Long Cannon for structures. That combination is our suggestion based on the official roles, not a tested build.
Diving and Underwater Resources for Ship Upgrades
Diving feeds your ship in two ways: treasure and materials. Start in shallow shipwrecks, improve your gear, and then head for deep-sea ruins. Deeper waters mean more contamination and more dangerous creatures.
| Resource | Where it comes from | What it does for the ship |
|---|---|---|
| Skyfire Gold | Mining meteors during Skyfire in Falling Ember | Crafts alloy ship structures and automated gathering facilities |
| Black Tide Fuel | Black Tide areas | Powers the Generator and the Fuel Propeller, raising their caps |
| Mist of Deceit Wicks | Mist Core Grass in Reefshadow Bay | Used for cannonballs and electricity facilities |
| Black Tide Clots | Black water near Sporocysts | Used for diving items and water circulation facilities |
| Bottled Raindrops | Defeating enemies and gathering crops in Time Rain | Used for water-based production and farming |
| Shell and Conch | Coast and tidal gathering | Used to build certain facilities |
| Ocean Trash | Gathering at sea | Smelted into plastic |
For safety, wear the Basic Diving Suit for basic resistance to ocean contamination, and apply Fish Oil Paste for temperature resistance. The Shark Diving Suit adds stronger protection and swimming speed. The Oxygen Tank activates automatically when oxygen runs out, the Inflatable Float helps you ascend fast and the Spear Gun handles underwater enemies. Bubble Fish release bubbles that refill oxygen, and an Anglerfish’s lure gives a safe haven from contamination.
Underwater teleport, cross-world return to ship and on-board respawn also help. Together, they make resupplying on your deck easier.
Ship Hazards: Giant Squid, Sharks and Haywire Zones
Not every danger is an enemy ship. According to the dev blog, a Giant Squid latches onto your ship with its tentacles, which leaves the ship immobile. Focus your fire on the tentacles, and the squid flees once they are severed. Sharks track their target and attack in bursts, so decide quickly whether to fight or flee. Jellyfish discharge electricity, so keep your distance when swimming.
Haywire Zones change the local rules. Mist of Deceit lowers temperature and limits vision, Black Tide drains Sanity, Time Rain speeds Energy and Hydration use and wears gear down, and Skyfire drops damaging meteors. Prepare protective items and dishes before you enter. Examples include Mist Core Clarity Soup, the Black Tide Protective Mask and Time-Reverse Cookies.
Calamity Tower, Voyagers and the Sea Calamity Zone
Your ship’s firepower matters most in three naval challenges.
- Calamity Tower: the fight revolves around tentacles and weak spots. Some tentacles have building protection, which is where ship cannons shine, and Phase II adds a harder Abyss version.
- Voyagers: you destroy their key facilities, board their decks and loot their holds. They get stronger as the sail progresses, and larger Voyager Fleets appear after Phase II.
- Sea Calamity Zone: in Phase II, you blast buildings with cannons, detonate naval mines early, destroy turret supports, break the shield core and then defeat the Calamity Hatchling.
These challenges also drive progress. Completing main-story Calamity Tower challenges unlocks corresponding gear crafting, so a stronger ship opens your next Tech tier.
Sunken Lord Strategy: What the Sea Fortress Does and Does Not Do
The Sunken Lord is the endgame boss. Its entrance platform rises when the server’s purification progress reaches 100%. The team then defeats the Calamity Hatchling in the entrance area, completes a door battle and goes deeper to challenge the Core.
Here is the key point for ship builders. The official description of the fight talks about corridors, multi-layered platforms, tentacle sieges, arrayed barrage, violent smashes and spreading lava that compresses safe space. It does not say you use your ship during the Core fight. Therefore, we cannot promise that a bigger fortress helps directly.
What the ship does is carry you there. Naval challenges raise purification progress and unlock gear, so a strong ship makes the road to the Sunken Lord faster. For the fight itself, the studio’s advice is to coordinate with companions and find openings during alternating assaults. We will update this section when players publish verified mechanics.
Timeline: Phases and Permanent Server
The scenario is a PvE experience with a core cycle of about 21 days.
| Stage | Length | Notes |
|---|---|---|
| Phase I | About 5 to 10 days | Shortens as purification progress advances |
| Phase II | 11 days | The Sunken Lord challenge unlocks as progress advances |
| Permanent server | Expected November 12, 2026 (PT) | Opens before the first servers close |
| Worlds per server | 12 | Set by the developers |
The studio notes that its preview is subject to change, so check the live version for values.
Tips and Common Mistakes
- Waiting for a Sea Fortress blueprint. It is the result of upgrades, not a single item.
- Skipping your design save. Saved designs let you rebuild quickly.
- Ignoring squid tentacles. Sever them or your ship stays stuck.
- Diving unprepared. Bring protection and watch your oxygen.
- Assuming the fortress wins the Sunken Lord fight. The official description is platform-based.
- Carrying valuables in PvP. In RaidZone, supplies on a sunken ship are not refunded.
Conclusion
The Once Human Sea Fortress is the end of a build-and-upgrade ladder that begins with a skiff. Add hull sections, deck workshops, stronger propulsion and a mix of cannons as Tech unlocks them, and dive for the materials that fuel those upgrades. Use your ship to clear Calamity Towers and Voyagers, then team up for the Sunken Lord once purification hits 100%.
FAQs
What is a Mobile Sea Fortress in Once Human?
It is the top end of the ship progression, a fully upgraded vessel that serves as a base, workshop and heavy artillery platform.
How do I upgrade my skiff?
Build a ship in Build Mode, then add hull parts, better propulsion, cannons and deck facilities as your Tech advances.
Which cannons should I use?
Use the Harpoon Cannon for sea creatures, the Long Cannon for buildings and the Rapid-fire Cannon for Calamity Tower weak spots.
What resources upgrade my ship?
Skyfire Gold crafts alloy structures, and Black Tide Fuel boosts Generators and Fuel Propellers.
When does the Sunken Lord unlock?
When the server’s purification progress reaches 100%.
When does the permanent server open?
The studio expects it to open on November 12, 2026 (PT).
For more updates visit: GAME SPEC
