Wow, great idea.
The one frustrating bit was going back to your surfboard without being able to control the direction. I get it, from a design perspective, the concept is cool, to punish the player who keeps throwing the board nonstop, but sometimes you get stuck and the only option is to wait for a hit or swim towards a fish. One idea would be to move the player automatically towards the board (not pressing space) and the player can only make some directional adjustments. Or allow the player more free form control while swimming but putting time pressure on life, like you have an air bar that depletes a little every frame you are not on the board, so you need to get back to it fast or take a hit.
Something I would exaggerate is the feedback when you get hit. Make the screen shake, animate the heart disappearing, give the player a more distinct sprite. I didn't even understand how my game was ending for a while
Had a great time, the design is very interesting and very suitable for a short arcady experience. Great work!