What dose buoyancy have to do with jumping?

Buoyancy is a condition in which a physical body's average density is equal to the density of the fluid in which it is immersed.

The buoyancy offsets the force of gravity that would otherwise cause the object to sink (if the body's density is greater than the density of the fluid in which it is immersed) or rise (if it's less). An object that has neutral buoyancy will neither sink nor rise. 

Hope this helps! xx