Friesiansam
A three-phase motor running on DC, are they sure about that?
Water pumps tend to use AC motors, with the AC provided by solid-state wizardry included in the pump. The rotor is a permanent magnet that's fully flooded by the coolant, so you don't need to seal a rotating shaft. Even fans are AC these days, commutators are pretty rare in PCs (one less thing to wear out, and the lack of conductive dust given off is desirable)