Starting from a "plie position" (heels together, knees bent outward, and bottom tucked under), jump as high as you can out of the water, returning to the plie as you land. The deeper you go in the water, the harder your quads and butt will have to work to propel you out of it.
Tip: To really work those inner thighs, go as quickly as you can with no rest at the top or bottom of the move. Ribbits are optional but totally encouraged.