The underwater illumination apparatus for use in nuclear facilities has a high pressure sodium arc lamp sealed to a stainless steel base with a flexible, radiation-resistant seal to provide a watertight and shock-absorbing connection. A wet-mateable base connector is attached to the base to permit connection to a lower cable. A light-transmissive, impact-resistant cover is formed around the base and the arc lamp. The cover has holes through which water can flow in and out to conduct heat away from the arc lamp. The combination of the above elements creates a modular unit which is replaced as a whole when the arc lamp burns out. The lower cable which provides power to the arc lamp is attached at its other end to a ballast power supply which is hermetically sealed in a stainless steel housing. Wet mateable connectors are attached at the inlet and outlet of the ballast power supply to attach to the lower cable and to the upper cable connection to a 120 VAC source. A number of units may be assembled with a single reflector to create a light ring.