I've finished the game, got all the items except one. This is the one to the right of the green shark fin. What and where is it?
It's the purple fin as they already said.
The trick to get it is to squeeze and keep yourself as high as you can in the screen, then just spam black_demons to kill sharks (I recommend to have the unicorn horn to do this).
If you have enough HP and HP regen you won't die till very late, when there're already 3 snakes on screen