Never played this kind of game, but it looks like Luftrausers. Took me a bit to understand that you could only heal by not shooting, but after that it wasn't too hard to beat the demo. I seemed to have missed the existence of the laser though. The waves were each fun in their own way, the last one felt a bit busy so I was mostly running. The spawning enemies right in your face was annoying. Felt a bit weird that there seemed to be no ceiling or walls to the arena. I was disappointed that the water kills you. The current slow down + preventing HP regen would be more than enough I think. I could complain about a bunch but it's mostly visuals/sounds.
Game looks cool, keep it up.