Honestly great. A couple of times while writing this review, I went back to play another round, because it's so fun.
The ships and mines appear close to the edge of the screen (luckily never so close to you that you'd have no time to react), perhaps they should appear just beyond for realism, but it's fine.
I also like the ship variety, although I'm not sure if the larger ships are any better than the smaller ones. The galleons look intimidating but they are (seemingly) just as fast as the small ships, which just makes them bigger targets. I like the "ghost ship"/"Black Pearl" kind, too, even though at first I thought it's a bug (it's single-coloured, and there's way too much smoke). Perhaps the larger ships should have a larger health pool (if it doesn't ruin the game's balancing).
The best strategy to deal with enemies is probably to lure them into mines, since with a single health point, confronting a ship directly is very risky, because it's nearly impossibly to dodge a cannonball when you're sailing towards it.
After you collect a bunch of powerups, it gets better.
I feel like you could have fun with the sea itself – it looks too calm. I was going to suggest adding a wave pattern texture, or some fake waves animation, but that would perhaps make it too distracting, since you really need to pay attention to every object onscreen.
But something like an occasional maelstrom pulling nearby things towards it could be interesting.
The cannonballs don't seem to be spawned in the muzzle, rather below it, so perhaps you should adjust it slightly.
Sometimes it felt bad if I had a cheap death after a particularly good round when I had to start all over again and I thought this could work as an 'upgrade game' to prolong the playtime, e.g. the money earned from each round would be used to buy permanent upgrades for the ship.
Anyway, thanks for this, it's a great little game.