That is just confusing. There is nothing to render over water so your frame rate should be good over that and is usually bad over land since you are rendering trees and clutter depending on your visual settings. All the water depth is set to -100 ft.
Ford island had no objects on it and I wasn't able to put any ships in Pearl Harbor since we no longer have static ships we can place right now. The only difference that Ford island had was that it was painted with the rock terrain instead of normal grass, farm, tree that the rest of the island island.
Now around Ford Island you do have a port, ammo factory, large airfield, town, and 2x2 refinery with an 8x4 mile area. So was it tanking by Hickam (A4) or more mid island by Wheeler (A19) which used the new 4x4 mile layout?