With air cooling, I use one 120mm 30 dBA fan with a 92mm rear fan - that's it. Hard to argue with that. I get this a lot. I use hose clamps on all fittings and cannot pull them off. With no extremes of heat, pressure or vibration, a water cooling system in a computer is in about as benign an environment as you can find.
Nothing is infallible, but the odds of catastrophic failure are very low.
If you are super concerned, use distilled water - doesn't conduct electricity. Not necessarily, but realistically - Yes. You can run an open case and keep the radiator outside the case, but I don't think this is a desirable long term solution. The alternative is to use a tub of water rather than a radiator, but if space or portability is an issue, this is not going to work. Plastic blocks are also coming on strong.