Thanks for your responses. The idea with jelly fish is actually genius in its obscurity, too bad there are none in our region. 
And as I said before, going and asking "you wanna piss on me?" is rather too straightforward. Although he could agree on that as a form of humiliation after the lost bet, asking him for humiliation (from his POV) without any context is not really an option.
I'm living with a straight flatmate. He doesn't know I'm gay and is a type who'd freak out after learning that I am. I don't want this to happen, yet I'm absolutely crazy about his dick. Considering the extent of his straightness, the chance of convincing him to dump his load on me is close to zero. However, I'm very much into watersports and I thought there could be a way to make him piss on me - for example in the shower.
The guy doesn't hesitate from taking a piss in the urinal, no matter if there are people around or not so he is not piss shy, but when we go to a biking or hiking trip, he never takes a leak, doesn't even want to go into the bushes for the minimum privacy. He said he wants a bathroom, that's why he always goes there when we reach some food venue.
I'm pretty sure he'd think of being soaked as a form of humiliation, so maybe some bet with "winner takes a leak on the loser" rule? I don't have to mention I'd be trying to do everything to lose. 
And if he notices I'm totally hard when he makes me all wet - well, we shall see. ;D
Any ideas for what could be the the bet? Maybe you tried something similar and if so then how it worked? Or maybe you have some other ideas how to convince him to soak me? 