It's got potential, but I see a couple of areas that could use improvement.
First, jump height varies, but it's not at all clear to me what causes that variation. All I can tell is that sometimes I can make a jump, and sometimes I can't. That's especially frustrating when jumping to escape a crab.
Second, in "Dive to the Utmost Depths", you've got a crab that kills the player in what seems like less than half a second after the level starts, if they don't move. It's easy to get dozens of deaths here before realizing what's happening. Maybe start the level with the game paused, and have everything start moving when the player hits a key? Also, it seemed like the motion of the fish wasn't the same every time, which makes it hard to learn how to avoid them.
Maybe I'm just not the intended audience of this type of game? It's definitely cute. I hope you get a lot of useful feedback!