Theoretically it would actually be hydrogen, with helium as a distant second, which are relatively abundant in space. That opens up questions of hydrogen embrittlement, which are a major stumbling block of hydrogen fuels on earth as well (that and most hydrogen fuels are made from cracking fossil fuels). Or harvesting hydrogen/helium reliably in space. Both problems that would have to be solved for long term space travel, using nuclear fusion. Not to mention solving the original overheating issue, which would also need to be solved for a spaceship larger than a lunar lander.

Of course, given a closed loop radiator based system, radiation efficiency depends on temperature, so molten salts may well be the way to go if you're not using pure elements. Water doesn't like being water all that much when it is maintained under pressures and temperatures that start with thousands of atmospheres and degrees.

So, to reiterate my original post with all of the downvotes, given current technology it's stupid. But then again, given current technology going to space itself it stupid and never worth the price tag. Spend enough doing stupid things and you figure out ways to make them less stupid.