An underwater lighting unit (1) for marine or other underwater use incorporates LEDs (13) as the light source and provides an array of LEDs (13) mounted against the back wall (11) of a metal or thermally conductive plastic housing (10) which utilizes the cooling effect of direct contact between the water and the housing to dissipate the heat generated by the LEDs (13) in use. The LEDs (13) each have an associated collimator (20) protected from contact with the water in which the unit (1) is to be immersed by a sealed glass screen (22), and each LED may be from one to three watts or more in power. There may be 30 or more LEDs (13) and associated collimators (20) in each lighting unit (1). The lighting unit (1) may be mounted in a cofferdam (2) of a marine vessel, directly against or slightly forwardly of the back wall of the cofferdam, or may be surface-mounted on the hull (3) of the vessel directly against or slightly spaced from the hull (3) and below the waterline.