2. How many underwater lights do I need?
As a rule of thumb we normally recommend the number of engines you have, plus one. So two underwater lights for a single. Three for twins (two outside, one in center). Four for trips, etc. Underwater lights are usually installed between each outboard, plus two on the outside.
Generally 1 light per 2 1/2 to 5 feet of beam at the transom will give a nice uniform glow at the stern. As far as depth, underwater lights should be installed 8″ to 16″ below the waterline.