Over water a plane in groundeffect will use slightly more energy than it would over land because even though water is incompressible that is effective only when it is completely constrained. Since water open to the environment at the top can be pushed inwards some of it will do that and this will cause a rise in the water just outside of the profile of the wings of the aircraft. Over a solid surface this would not happen. Similar to how riding a bicycle with springs costs more energy than one without.
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Over water a plane in groundeffect will use slightly more energy than it would over land because even though water is incompressible that is effective only when it is completely constrained. Since water open to the environment at the top can be pushed inwards some of it will do that and this will cause a rise in the water just outside of the profile of the wings of the aircraft. Over a solid surface this would not happen. Similar to how riding a bicycle with springs costs more energy than one without.