Not bad for a first game! I like the art for the tile sets and I like how the character changes into a fish when they touch the water. The player character controls well.
The game could use more sounds effects and music, though I can understand those being lower priority when you're aiming to make all the assets yourself. I noticed the ladders are kind of buggy. I remember struggling a lot with programming ladders when I first started out.
Does the game have an ending? I ended up in this room in the level with the moving platforms and couldn't progress at all. Wasn't sure if I missed something. Overall, nice job on your first game and making the assets yourself!