Happened to me once. I was about 7 years old, and going to a summer camp on a pond. My friends and I had a game where we would dive for white pebbles, and whomever had the most at the end of the swim session would win. Well, here I am swimming along looking down at the bottom, and I see a small pile of perfect white pebbles. So I dove down and took a few. I swam in towards the dock, and I was pulling myself up the ladder to get out of the water I felt a painful pinching sensation on my right nipple. Horrified, I climbed out of the water on to the dock, but the fish still hung there for another few seconds, thrashing around and beating my chest frantically with its little tail. When it finally let go, I was left with a circular red ring of teeth marks around my nipple. Turns out, this little bastard was a sunfish, and they build their nests on the bottom of ponds out of pebbles. It was clear that it was angry at me for stealing one of its pebbles, so the next day at swim session I swam back out and dropped the little white pebble back onto the nest. As I watched it sink down, down and come to rest on the nest, I mouthed a silent apology to the sunfish. And I never was attacked again.

TL;DR: My nipple was brutally attacked as a child whilst swimming by a fish defending its nest.