Fun little game! I don't really see how it falls into the main theme all that much just from the side of gameplay, but yeah! It seems like there's a few ways you could go with this thing in the future. I like the idea that the fish is piloting the uh, "fish tank" and that it's using up its own water, but as-is the only penalty for using up the water is just instant death when you use up the last bit. Maybe if you were to expand upon this in the future, you could have it so that the water is a resource that can be used for multiple things?
Also, I found the different colored water an interesting idea, although possibly could be a bit better implemented in the future. Yellow didn't seem to do anything, red seemed to just be a slow bullet, and the green bouncing bullet just reflected back to you, not bouncing off the walls with any other sort of reflection (although that requires some weird math if I recall)
Still, I like it! Thank you!