Mine is a fun story. Back when I was in college, I was friends with a lot of japanese students. I took them all out to my parents' place with me one day, which happens to be a house boat on a relatively large lake. And I wasn't aware of this, but apparently lakes this large are uncommon where these girls were from, and they were totally awed by the size of it. I tried to get them to swim with me, but they weren't having any of it, said the water was too deep and too green. So I swam by myself. And I've never had swimming lessons or anything, but I've been around water always, so I guess I have a really natural swimming motion? And one of the girls tells me, "Ohmigosh, you aren't even human, you're a kappa!" and the name sort of stuck.
In explanation for people missing out on japanese mythology, a kappa is a water monster kind of like a turtle with a duck's beak, and a bowl in its head full of water that will die if you spill it.