1. Give yourself elytra and fireworks.
2. Using the elytra, fly into the water (preferably into the ocean).
3. If you will start swimming (only without the sprint key!), you will see yourself in a swimming-like position, but actually you are still in elytra fly animation. You can even use fireworks to fly faster in the water, or to get out of it.

How should it work: It should turn off elytra flying in the water. Imo it's a fast-travelling in water.