For a marine aquarium it is typical to not have any glass covers on the top of the tank.  The reason for this is to do with their presence restricting gas exchange and absorbing light between the bulbs and the tank.

Having glass covers can:

•  prevent fish from jumping out of the tank and going carpet surfing.  A well designed tank hood will also provide this protection.
• reduces evaporation, which may be an factor in warm, dry regions.
• protects equipment above the tank from salt spray and splashes.
• can reduce noise levels.
  

 Not having glass covers can:

• improve gas exchange as having them will hold the same air in contact with the water surface.
• increase amount of light that reaches the tank form the light bulbs.  Not that significant with clean glass, but as it builds up dust and salt spray, it can absorb a lot of light, reducing the intenisty significantly.
• increase evaporation, which will allow for better temperature control in the warmer months and increase amount of evaporation top up that can be added, important if using calcium hydroxide for alkalinity and calcium maintenence.
• eliminate the need to clean and avoid breakage of the glass.

It is definitely recommended though to have glass shields between the water and metal halide bulbs.  This will ensure protection from explosive failure of the bulbs, stopping the bulbs from being splashed with water, and for some metal halides act as a UV filter (some types don't have one).

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