Kevin: There is a section in Prouty where he reports on a series of tests
conducted at Cal Poly that claimed to demonstrate no change in hover power
required over water. I've put that in the magazine once and received a
bunch of howls from helo pilots claiming other wise. The Cal Poly guys did
have some instrumentation with numbers to support their argument.??

Nice that somebody mentions!

"Mike Baker and Jonathan Scarcello made ground effect measurements over
astroturf and over water. The astroturf results tend to refute the
pilots' observations. The ground effect at half a rotor diameter was
roughly 30% stronger than over the smooth solid surface. Over water,
however, there was _little_ measurable difference"

Prouty in "More helicopter aerodynamics" Chapter 2.

I got kicked in the ass on mentioning that towards some of my CFIs.
And I noticed that I need little more power in hover over tall
grass/bushes. However, that's not astroturf.
I never hovered over water.