Many factors will govern the placement or your underwater lights, including the location of a flat mounting surface, the ability to safely run power to the location, and the location of engines, struts, trim tabs, etc. which may obstruct the light output.
Although Lumitec underwater lights can be run above water, they perform best when completely submerged 200mm to 400mm below the water. For inboard boats: generally speaking 1 light for every 750 to 1500mm of beam provides good results. For outboard and stern dive vessels, good results are generally achieved by installing lights outboard of engines, and between engines. For example, a twin engine vessel would use 3 lights, a triple engine vessel would use 4 lights, etc.