since when does oil evaporates like water?
lol, I'm pretty sure it's not the actually oil that is evaporating into the environment, but rather the Corexit's fumes. As it leaves the body of water is then that it changes into a gas.
I'm not sure how hard that part is to understand, (not being sarcastic or condescending), btw.
But, you can use many examples, to test this theory...