I really enjoyed this! It felt very satisfying to clear a level.
My advice for improving the game would be to make the "is grounded" check more forgiving. For example, maybe check from the back of the penguin rather than the centre. Sometimes I jumped, landed right on the end of the next platform, then couldn't jump again. That was frustrating.
I enjoyed the attention to detail. The screenshake when you hit a wall is nice, and the "splash" sound when you fall off the bottom really surprised me. I died from walls, fish and sealions all the time, but I only hit the water once 😂.