When your head is not fully in water, but your legs are, and you sprint. You will not trigger the swimming animation, and when going into deeper water, you don't fall until you stop sprinting! If you start sprinting in deep water, but don't fully fall in, this happens too.

To reproduce:

1: Go into 1 block tall water
2: Start sprinting
3: Sprint into deeper water, without stopping
4: Look down
5: Realize your not going down