So this is not in response to other topics looking for something else to play until SoT launches. Most of these were pretty mediocre. Let me save you some time if you were looking.
The Legend of Pirates Online:
This is a private server of the defunct Disney PotC game. Disney appears to have no problem with it for some reason, so I'm listing it. This game sucks, BUT it has the Pirates soundtrack built in so you don't have to hum it.
Salt:
Sailor Minecraft. Not worth the $20 vs Minecraft with mods. It is a co-op sandbox sailing game with skeletons to shoot. Pretty much worse than every other MC ripoff in every other way.
Two Worlds 2 with expansions:
Probably the most fun I've had with a bad game. This game has co-op chapters, a few of which let you take sail. Just imagine attacking an island, but instead of skeleton pirates, it's full of kobolds gesturing at you with spears. Think Wind Waker in a LOTR-like world. With bad controls.
Guns of Icarus Online (any version)
This is the most like playing SoT, if you remove everything but combat. The community is rapidly dying, but it is a first person game that involves sailing (airships) cooperatively. This is actually a solid game, I'm just including it in this list of shame because it's mostly dead. If you buy it cheap on Steam for yourself and 3 pals, expect to just fly around blowing up AI ships over and over for a day or two.
Uncharted Waters Online
If you wish nautical games were more about buying barrels of grain low and selling them high, this is the game for you. It plays a bit like Eve Online, definitely more of an RPG than an action game. In fact there is no action. But there are randomized status effects, so you can suddenly have rats on your boat in the middle of the open ocean, 5 days from port.
Blackwake:
This is actually a pretty awesome game. It is all hardcore PVP all the time and plays not completely unlike SoT. It's also everything casuals don't like about SoT x1000 though. I picked it up without too much trouble, but do not be the person with a c****y PC just getting started, you will be hated by all. I definitely saw people hazed and I'm not too down with that.
This is my running list so far. It's tough to wait in a world without good pirate multiplayer games.
