Remove the cord from the center of the lamp. Seal the hole at the bottem and remove the metal tube from the center. Rewire the light so the cord does not come in contact with the water. You will aslo want to seal the light fixture to prevent the humidity from affecting it. Find a switch that can be place on the cord. As long as you keep the water from getting to the electricity, you should be fine. They install lights above fish thanks all the time.
Enjoy
Psycoman