What is the item to the right of the green shark fin? [duplicate]

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

I saw the answer in another thread on here. Its the Purple Fin. You have to go through the sea for a very long time and end up at a point where you basically have three sharks and two snakes on screen at any one time, so you need mega health to get there.