How are nuclear submarines heated?

Heating isn't really a problem, as there's enough heat from just about every operational system aboard that's turned on. The engineering spaces alone can reach above 120° F if the ventilation and AC is off.

Every major system, from the engineering systems, all electronic systems, even the galley ovens and stoves generate heat. The real problem is keeping everything cool, and for that reason the boats are cooled by huge AC systems and fans that run throughout the ship.

During a casualty drill, the fans are turned off (e.g., to deny the spread of smoke or contaminants from a fire), and you find out very quickly just how much heat is being produced by all the onboard systems.

The only real spot in the ocean where it gets a bit chilly is in the Arctic, where the water temperature is a constant 28° F. But there's always a nice comfy fan blowing heat from some equipment to warm you up.