It's either water or fire... With fire, you generate heat, and you can burn stuff, though it would be very obvious, since you're not exactly pyrokinetic (wah no inconspicuous spontaneous combustion!), but you can still generate heat... But with water, you can do all sorts of things with it, all sorts of things... And there's the whole "I won't die of thirst or by drowning" thing. And after reading all the other water threads, I feel myself being seduced... But dang lightning is awesome!
I'd choose with whatever the coin lands. And apparently, it landed with tails—fire.