I tested it on Linux Mint and it seemed to work fine.
One thing that confused me a bit was whether the game is meant to have a lot of negative space on the left and right of the screen? I wasn't sure if it was meant to be this way or was becoming confused by my second monitor being vertical. I hope that makes sense, for some odd reason; I wasn't able to take a screenshot.
If so; have you considered having the viewports on the left and right side, taking up more of the screen?
I'm hoping the final version will allow for 2 controllers. I just played it by myself, but I imagine it would be more fun playing it against someone if we were both using controllers.
I noticed that sometimes the collision for the treasure chest seems a bit off, where driving right into it wouldn't pick it up, but touching its side seemed to work. I originally thought that they couldn't be picked up.
I also found it quite easy to get stuck on land, or on the corner of land... I'm not sure how you handle collision, but I think it would be more fun if you slowly slid back into the water, so you still get penalised, but you don't just get stuck.
I think the game could be quite fun with a bit of work. Off the to of my head, I'd suggest:
Adding momentum. The boats currently feel quite stiff, but I think allowing accumulative velocity with slow damping would make them feel a bit more like they're drifting in the water. Being hit, rammed into or being close to an explosion could also nudge the velocity, giving it a little more chaotic fun.
Power-ups! I imagine having short bursts of speed or spread shots could be fun, but if the momentum is added; you could also add "power-downs", which you could nudge the other player into.
Visual feedback. Maybe making the boats rock back and forward when hit, sway when turning and so on. Maybe a little fire on the boat when it has been hit and a screen shake. I just think it would make it a little more visually interesting.
Hopefully some of that was helpful, sorry if it was a bit all over the place; I'm recovering from the flu.... somewhat dazed. I'd actually like to see where this goes, I love playing local multiplayer with my partner and I think she might actually like playing a game about pirate ships.
Best of luck :)