Abstract: A portable, submersible heater for heating water in an aquaculture water container by resting the heater on a bottom surface of the container includes an electric resistance heater enclosed in a cylindrically shaped heater body, a pair of standoff plates fixed in proximity of either end of the heater body for preventing contact between the heater body and the bottom surface of the aquaculture container regardless of the orientation of the heater within the container and a power cord connected to the resistance heater and entering the heater body through a water tight connector at a first end of the heater body. The cord has an external cord section located outside the heater body and an internal cord section located within the heater body. A thermal protection device in electrical communication with the resistance heater allows the heater to cool in when the water level falls below the heater while automatically restoring power to the heater after a power outage.