i think, it's client-side only, but anyway:
1. start to mount.
2. when you are almost mounted,disable your internet-connection.
3. jump into the water.
4. enable your internet-connection.
(emotes like /sit or /dance don't work ;_; )
/edit: look, what happens, when you jump, or swim back-/forwards. this effect stays until you press left or right on your keyboard (you can use your mouse). this works also on land