An underwater swimming pool light is a sealed unit designed to keep water from making contact with the bulb and electrical socket inside. If either the lens gasket at the front of the unit or the compression nut at the rear of the unit leaks, water will enter the interior of the light, and the light will fail. Here's how to waterproof swimming pool lights.
Make sure the power to the light is off before beginning any work on it. If the light operates on a timer, remove the trippers on the time clock. It's also a good idea to turn off the circuit breaker that controls the light as an extra precaution.
Remove the light from the niche in the pool wall. Pool lights are usually only about 18 inches below the water surface, so you probably won't have to lower the water level to access it from the pool deck. A single screw at the top of the light ring holds the light in place. Use the screwdriver to remove the screw, pry the light unit out of the niche and pull the unit up onto the deck. There should be enough extra electrical cord coiled in the niche behind the light to allow you to the lay the light on the deck.
Notice that the light unit is held together by a stainless steel clamp surrounding the light lens. The light lens has a rubber lens gasket that fits over the outside edge. The clamp fits over the gasket and the ridge of the back housing, holding the two parts snugly together. Loosen the clamp nut with an open end box wrench and remove the clamp. Separate the lens and gasket from the rear housing.
Examine the inside of the light unit. Completely drain any water you find in the unit. Remove the bulb. The light socket and the electrical connections are sealed in the rear of the stainless steel housing with a layer of amber epoxy. Inspect the socket for corrosion or deterioration, and carefully examine the epoxy layer behind it. If the socket looks bad or if there are any cracks or checks in the epoxy layer, do not attempt a repair. Replace the entire light unit.
Dry the unit out completely if the socket and epoxy layer are in good condition. Use a towel to get the inside completely dry and let it sit on the deck for an hour or so to be sure the inside of the socket is also dry.
Inspect the compression nut and gasket that holds the electrical cord to the back of the unit. Use the box wrench to be sure the nut is tight and the gasket is properly seated to prevent any water from leaking in through this connection.
Carefully screw in a new 500-watt pool light bulb. Remove the old light lens gasket and mount the new one onto the outer rim of the light lens. Set the lens into the groove of the stainless steel housing and be sure the gasket seats properly all the way around. Hold the lens and housing together and reinstall the clamp. Screw the clamp on snugly.
Put the light unit back in the water and hold it a foot or so below the surface. A sealed unit will float to the surface. Push the unit underwater again and hold it down for a minute or so. Watch for air bubbles escaping from the unit. If you see a stream of bubbles, it means there's still a leak, and you will need to re-seat and re-tighten the clamp. If no air bubbles appear, push the cord and light unit back into the niche and secure with the holding screw.