[NicknamedBob]

"If you threw the rock from the boat into the pond, would the water level raise or lower?"

At the beginning of the experiment, the weight of the rock is displacing an equivalent weight of water.

At the end of the experiment, the volume of the rock is displacing an equivalent volume of water.

Less water will be displaced, unless the rock is pumice.

[ArGee]

And there you go.

[Darksheare]

If the objects connected to the rocks wriggle and complain, there nsy be a miniscule loss of water volume through dispersal onto the surrounding area.