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You also have to consider salt spray/water splashing. I think it's pretty standard to mount them approximately 10 - 12" off the surface of the water. Unless of course, you have a shield over your halides, then you could put them closer...although like Phil said you would have to worry about heat.
I just installed a retrofit kit into my canopy and it was a pain because all-glass canopies were not designed for a retrofit. But it worked out great and i have mine at least 10in off the water. they do cause alot of heat so you might want to install a couple of fans.