Really love the concept! I loved the feeling of exploration as you go deeper through the tunnels and the music really adds to the atmosphere (although the shooting sounds seems a little out of place). The sprites are really well made although I think it would be better to render them as sharp instead of blurry so we can see the pixel art properly. I like the fact that the movement has momentum making it feel more like a proper underwater vessel, although I think it's too slow to stop - if you're travelling at moderate speed it's almost impossible to stop in time when you see a large enemy.
Overall a really good start on an interesting take on a shoot 'em up, I'd love to see this taken further!