Suffers from a lack of player testing.Gem in rough
It needs a tutorial level, to explain the basic ways to navigate.
It needs a faster loading time for the harpoon. It's frustrating waiting for the magic harpoon to navigate back to your hand.
The enemies should be smooth, not spastic. It's boring waiting for them to slowly move out of the way of magical fish. It doesn't particularly add to the difficulty either.
It also wouldn't hurt to have some sort of pull up mechanism for the harpoon. The controls are rough, to say the least, and anything that would make the game feel smoother would be good.
Maybe some sort of story, or power ups? I didn't feel any desire to finish it. Did the first level. Then... nothing happened that held me. The difficulty is mild.
Your description is misleading. The first major enemies you meet can't be impaled by your harpoon. Thus, there's no satisfaction in murdering random individuals you meet. Murdering random individuals is at the core of any game's fun.