Helidrvr,
Your experiment is a very good one but the conditions are not the same.
When over deck/sea half of the disc is experiencing dissimilar ground effect but on a slope it varies constantly.
Over the Deck there will be ground effect for 1/2 of the disc and over the sea there will be none for 1/2 of the disc (depending on height and the effect of the water). The blades will experience the resulting differences in ground effect earlier and indeed, if alongside the ship heading to the stern (for instance) it will drift to sea.
Not so with a slope though, different game altogether!