On February 15th, Unearned Bounty [Steam] a free-for-all multiplayer naval combat game will release on Steam with Linux support, it will also be free.
While the game is planned to be free, I've read that the only items you will be able to purchase are entirely cosmetic, so winning will still be down to your skill.
I'm really happy to see more naval combat games, we had a bit of a drought up until the last year or two when more developers seem to be keen to do them.
In Unearned Bounty each ship will be slightly unique, with each having their own passive and active ability. They said they're trying to make it more like League of Legends and less like World of Tanks/Warships. So it will be very interesting to give it a go when it releases, could be pretty fun, the trailer makes it seems pretty decent.
The map system sounds good too, instead of coming up against the end of the world you wrap around and go back through it, no flat earth shenanigans here.
Thanks for the tip, ronnoc!