Small, small, small, why does the balloon fall so fast after falling into the water?

In hot water, the balloon will expand, and in cold water, the balloon will contract. Because the balloon's expansion and contraction are caused by the increase or decrease of gas volume, it shows that the gas volume increases with the increase of temperature and decreases with the decrease of temperature, that is, it expands with heat and contracts with cold. If there is no balloon in the experiment, the most likely reason is the poor air tightness of the experimental device.