I really liked this game! I've seen it a couple of times in Twitter before actually playing it. You absolutely NAILED the art style. It's "simple", but it's beautiful!
The music is cool, but gets a bit monotonous (since it's the same song over and over again). I like how the song has a low-pass filter under water.
I'm not a fan of jumping with Z, tho. I think the up arrow key would be a better fit. It's probably better using a controller, but I didn't have one at hand.
I'm excited about this game, and hopefully coins, upgrades, etc will make it even more exciting!