I always hold my MH about 8" off the water, when I can, to help with heat and still allow access into the tank. You don't lose much light by raising, but you allow it to spread a lot more (reducing localized intensity in the tank). Water is the big light killing medium. Think about it in terms of the sun. Its' light travels a long distance but the reduction in intensity from the edge of the atmosphere to the water surface is small compared to how fast it loses intensity in water depth.