There is no hard and fast rule as to the distance between the MH and water surface.
Dependin on the reflector type, you will be getting a wider light spread is the MH is placed further distance from surface compromising light intensity.
Nearer water surface, you'll be getting heavier intensity and more focused light source. Place your fixture according to your reefs' needs.
And please do take note of salt spray as these could easily limit the performance of even the highest quality reflectors.