When you swim down a waterfall, and fall out of the flow of water and land on dry ground, the swimming animation stays on.

The same thing happens when you swim out of flowing water onto dry ground. The swimming animation stays on and you stay only one block high.

Releasing control, jumping, or mining under yourself does nothing, and the animation stays on. The only way to get the animation off is to go to water and start actually swimming again, then stand up like normal.