You would have to include a water reservoir in the vehicle and that adds weight. You'd also have to find suitable parts to create such as system which is not common for RC cars and trucks. An RCgroups.com discussion offers up some ways to create such a system. An RCUniverse.com forum thread talks about it as well. A fully-enclosed RC car or truck might benefit from such a system if it's unable to get adequate airflow to keep the engine cool. A slow-moving rock crawler is another instance where it might help — because it might not move around enough to get the air flowing enough to cool the engine.
In either case you still have the weight issue and the need to pretty much build the system from scratch or adapt a water-cooled system such as those used in some nitro boats. That's a lot of time and money invested in something that may or may not pay off. It's really more of a "Wow, that's cool" factor. From what I understand, the only thing that would be useful is that you might be able to run a tad bit longer on a tank of fuel by leaning the mixture a tiny bit and have the water-cooling system keep your nitro engine at a good, safe operating temperature. Even then I wouldn't want to risk damaging a good nitro engine that in some cases cost an arm and a leg to buy or rebuild.
Bottom line: It can be done but it's not necessarily something worth doing.