There are many things that could allow for such an effect. Basically anything that is heavy enough to counteract the boyancy effect of water, and is also able to provide a liquid seal.

A ball of apoxy would probably do it. A very large marble would, as would a round piece of rubber.

You've lots of possibilities.

A tampon.

is it a spong

Think of something that is small enough to fit in a bottle, will absorb enough water to expand and plug the opening, that floats on water.