Survey String: You see a fort has been constructed on a high bluff overlooking the sea.
Vmap appearance: F on the vmap with a 2-3 room dock formed by #. You will likely hear from them before you see them. For flavor, the huaka sea fort commander will speak in their own language.
Details
Before reaching them, it is advisable to sink as many of the smaller ships around them as possible (caravals, drakkars and dromonds) to prevent being overwhelmed. Think of them like an advanced version of pirate hideouts. You do not have to kill all of the pirates surrounding them before attacking a sea fort. It is possible to get close enough to a sea fort while keeping a safe distance away from galleons and bigger pirate ships by navigating carefully. Needless to say, the intended design appears to be meant for a group effort of players with an actual fleet of ships instead of soloing, but soloing can be done by the tenacious/crazy.
Be aware of your surroundings, even for experienced sailors and pirate hunters. Note the escape routes, and try to keep hull above 75%. If your max speed is impeded due to hull getting too low, you may not be able to get back through churning whitecap rooms (depending on whether your route back to a dock has these).
Unlike regular pirates, they will not issue a warning prior to attacking. They are stationary strongholds that will attack within a range as far as "brief distance away" on SHIP LOOKOUT. It is unknown if they can target multiple player ships simultaneously - to be investigated. Difficulty wise however, sea forts are on par with super dreadnaught pirates. The usual pirate hunting strategies apply.
In addition to normal pirate weapon attacks (cannon balls, ballista, fire-launchers and harpoons), they will attack with a unique weapon called "cursed grimoire" which functions similar to how web filter used to. When affected by 'cursed grimoire jinx' your crew will occasionally fail to take your orders (whether it's firing your weapons or moving the ship). So if you want to ship fire fire-launcher for instance, note if your crew failed to take that firing order and ship fire again. Or if you tried to move and it fails, try again.
The lines are:
An internal component ratchets and clicks helplessly, malfunctioning due to the jinx.
The crew appears to be struggling to perform their duties!
Tip: The huaka sea fort has acid shores nearby (yellow dots in 16-bit colored vmap). These are safe zones for ship (not SPK) if you need a quick retreat.
Loot
Salvaging a sunken sea fort will yield: standard pirate loot (ammunition and gold-ingots), uncut rubies and uncut emeralds. Potentially lots of these (I have received 90 emeralds from one sea fort and 180 rubies from another). Make sure there is enough cargo space to salvage, prioritize what to keep and discard or bring friends.
Sinking a sea fort will add a crediting line in gossip to the person who lands the sinking weapon hit.
a sea fort commander reports: player has sunk a Labor sea fort in the Sigil Underground. a sea fort commander reports: player has sunk a Hegemons sea fort in the Sigil Underground. a sea fort commander reports: player has sunk a Huaka sea fort in the Sigil Underground.