My policy is the closer to the water the better with the deciding factor being how often I feel like cleaning the saltwater splatter that accumulates on the bulbs over time. The closer they are to the water, the more light the aquarium gets, but the faster they get dirty. It depends alot on your setup too. If you have a lot of turbulence on the top you tend to kick up more spash and you have to clean your bulbs more often. At 8 inches mine stay clean.
Heat is another consideration. If you run a closed canopy and heat is of concern, you might want to put them higher to help with that.
I used to run two 175's about 3 inches off the water over a 55 gallon closed canopy. The bulbs needed to be cleaned frequently at that height because they tended to catch more splatter. A paper towel with a little vinegar works good for that. Heat was not an issue since I ran a small fan when the lights were on and the tank was in the cool basement.