Agreed. There is probably no practical difference or even anything that the typical boater could measure (though I bet NASA could measure it).
I'm guessing that the difference in efficiency would be most apparent in a displacement hull or at least when a planing hull is not on plane. The boat would sit higher in the water and therefore have less to push out of its way. Somewhat similar to decreasing the rolling resistance of a tire by inflating it more.
Still, a mitigating factor could be that the smaller volume of water that has to be pushed out of the way would also require more energy to move due to its added density. Who knows.
Truly a topic that I wouldn't give a second thought to if my boat wasn't stuck under a tarp... :-)
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