What happens to the sea level when an object floats in it? Of course it rises.
What happens to the sea level when a passenger in a boat throws an object out of the boat, so that the object floats? The answer lies in Archimedes Principle, which states, 'the upthrust is equal to the weight of fluid displaced'
The weight of the boat plus object is the same hwether or not the object is in the boat, so the weight of water displaced, and the upthrust is the same.
The sea level does not change.
If the object is thrown out and sinks, then the weight of water displaced after it is thrown is less than before, since when the object is sinking, it only displaces its own volume of water, which weighs less than the weight of the object. In this case, the sea level falls.