An illumination apparatus which is, in the preferred embodiment, adapted to be mounted in a through-hull configuration in a marine vessel, and which is comprised of a housing, which is preferably cylindrically shaped, surrounding a light bulb and light bulb socket, or terminal block, a reflector, first adjustment structure for moving the light bulb and terminal block linearly relative to the reflector to adjust the sweep of emitted light, and second adjustment structure for altering the angular position of the light bulb, terminal block and reflector relative to the housing to adjust the angle of incidence of the emitted light.