This is a two part poser.
While in he boat; the weight will displace a volume of water equal to it’s weight.
When tossed overboard, it will displace a volume of water equal to it’s volume.
So, when the weight is removed from boat, the boat will rise in the water, and the water level will go down.
When the weight then enters the water, it will go up; but not enough to return to original level when weight was in boat.
Thus, the water goes down.
(We really don’t need a weight. Just consider that the boat sank; instead.)
If you were paying attention; you’d have noticed WHY the oceans are (Scary alert!!) rising.
So many more boats are floating in the water!