I've actually wondered about the over water stuff myself. It's been
drilled into my head that hover IGO over water takes more power, but
other than some surface wave action dissipating the downwash, the
water is, in effect, a solid surface. (Ever jumped off a 40' cliff
into water? It spanks you pretty hard!)

That's just the speed. I remember a nice program on television where
some guy had two pieces of pork fat and a plate. The first piece of fat
was smeared over the plate, the second one loaded into a gun and shot
the plate to pieces.

Actually, when you hover above the water, you deform the water, forming
a shallow pit. If the helicopter would suddenly disappear in a puff of
magic, the water would flow back into it. I think it is this deformation
that dissipates the energy.

Best regards