It's not enough to cut slits in your neck. You'll need to cut slits in your neck AND expose yourself to some toxic waste. Preferably as it goes into the ocean. Can you find some big pipe spilling toxic waste into the ocean? That should do the trick.

Or you could try injecting yourself with fish DNA. I'm not sure exactly how that works; you might have to expose yourself to some radiation, too. So maybe try injecting some fish DNA into your body as you get trapped inside a live nuclear reactor. That should give you gills, too.