I honestly thought the developer had forgotten about this, but apparently not. Pirate action RPG Tempest [Steam] has a fresh Linux beta.
Currently though, they've messed it up a bit as it seems you can't actually download it. I've sent them a message through multiple channels about fixing it. Hopefully they will do so soon! You can find the new announcement on Steam here.
It does look quite good, so hopefully we will be able to give it some impressions once it's fully working and out of beta. There's simply not enough good naval combat games on Linux, so it might fill a nice gap.
It has single-player and multi-player support, with fully real-time gameplay in a big open world. You're not just taking on other ships, there's also all sorts of sea-creatures to take down too.
Thanks for the tip, robvv!