It's not dangerous if done correctly. The main danger I can see is if the pump fails for some reason and the water stops moving through the system, it would fry your chip. Most pumps have a high MTBF rating, especially the aquarium ones. It works the same way the watercooling system in your car works. Water runs through the waterblock thats on your processor then to your pump/reservior, out to the radiator where a fan draws air throught the fins to cool the water then back to your waterblock to pickup some more heat from the CPU.