I like the look and feel of it. It's worth noting, though, that when the boat rotates, it does not conform to the pixel grid of the gameboy, but rotates all of the pixels it is made of instead.
From a gameplay perspective, I'd say that it doesn't really seem like there's a way the player can engage with the turrets without either taking damage or just shooting at them from out of range. This would probably be improved by improving the player's maneuverability in some way, like allowing them to move backwards.
Anyway, it's a neat little game, nice job :)