In a water pump for pumping coolant in a low temperature and a high temperature circuit with low temperature and high temperature housings each including a spiral flow guide structure, and a single rotor disposed in the joined housings and having a low temperature rotor part for pumping coolant through the low temperature circuit and a high temperature rotor part for pumping coolant through the high temperature circuit, the housings are joined with a heat isolating structure disposed between the flow guide structures of the high temperature and the low temperature housings to limit heat transfer from the high temperature coolant pumped through the high temperature housing to the low temperature coolant pumped through the low temperature housing.