“Does the supported weight change when there is a boat directly overhead?”
Archimedes has the answers you seek. A boat floats by definition because it is displacing an equal amount of water vs the total weight of the boat. I say weight and not mass as weight is mass plus gravity which is the actual physics component governing displacement in earth’s gravitational field. A boat of equal mass would float higher in the water and displace less fluid on the moon due to the moon having one quarter the gravitational pull of earth.
So are you saying the supported load does or does not change with passing boats?