While it may be logical that submersible units do not exchange air with the atmosphere and therefore should not have condensation formation, there is another consideration. When you energize a space heater and make the motor internal temperature warmer than the outside it helps to prevent entrance of moisture-laden air but it also drives out some of the air that contains moisture. If the motor is sealed there is nowhere for that air to go so the space heater cannot accomplish one of its tasks. The effectiveness of the space heater is reduced, though not completely eliminated.