To Reproduce:

1. Fly using an Elytra
2. Aim down while flying at a large body of water (like an ocean) at an angle so that the water will stop you before you hit the bottom (blocks). The angle is very flexible. As long as you do not hit blocks, this bug will/should occur.
3. You will be able to move around in water, but the elytra will stay active, and you will be doing the elytra flying animations instead of the swimming/upright in water animations.
4. If you exit the water, for a short time, you will still have the flying animation, but flying will be somewhat broken (you are not able to boost with fireworks). I am not completely sure about this.

Expected Result:

Elytra deactivates when you hit water, and you are able to trigger the swimming animation by pressing w twice.

Actual result:

Elytra stays active in water until you hit a block (like the bottom of the ocean). After you hit the block, you will turn upright, be able to swim correctly, and everything will return back to normal.

The images attatched show the problem.