From the same creators as Dark Orbit, and also an online flash based MMO style game, Seafight takes you upon the high seas as the Captain of a pirate ship. Sail the seas searching for treasure, or fight other ships.

It has a look and feel similar to the classic Pirates! game and while I've not played this one yet it looks like it could be fun.

Again similar to Space Orbit you can form Guilds and join forces against other players.

Take a look at http://en.bigpoint.com/games/seafight/