Note that efficiency of a thermal power device is dependent on the Δ T: the temperature difference between the heat source and the cooling source. Without a good cooling source the efficiency sucks.
So being in a northern climate next to a cool body of water is great. Unless things start heating up. Then they have to cut back on the reactor power because there isn’t enough cold water to keep it properly cool. And the warmer the water gets, the worse those situations will become.
This is a restriction on some thermal source locations (natural or man made): How are you going to cool the steam back down?