>>715015707>>715015930>>715016115>>715016290It floats so obviously it's empty and full of gases that are lighter than air.
It can also go underwater because it can expel these gases and instead fill most of his body, where the gas is stored, with water thus when combined with his actual body weight he's denser than water and sinks down.
Therefore he can control his body density by expelling or producing said natural gases as needed and thus how well he can sink or float.
I don't really know jack shit about pokemon but that's my game theory (tm).