I've used krylon fusion for well over a year without any problems under water. I'm using it right now on my return plumbing that is visible from the front of the tank (most of that being under water). The key is to let it cure for 48 hours or more.
"What, exactly, is the internet? Basically it is a global network exchanging digitized data in such a way that any computer, anywhere, that is equipped with a device called a 'modem', can make a noise like a duck choking on a kazoo." - Dave Barry