The bug
While gliding into a body of water or lava, your player continues to glide instead of swim.
This means you can still use fireworks in order to accelerate and get out of the water or lava.
It also means that when you reach the surface of the water, it is impossible to get the player model back in upright position until you touch the surface of a block.
Code analysis by @unknown here.
Related issues
is duplicated by
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Can confirm.

Can confirm in 1.9pre-1

also in 1.9pre-2

Can confirm. also isn't fixed in Minecraft 1.9.1 pre release

Important to mention that elytra is loosing durability while swimming like that.
Confirmed for 1.9.1-pre1

Confirmed for 1.9.1-pre2

Not only is the model stuck, but the game still thinks you are flying and the elytra still gets damaged until you get out of water.

The flight also starts again when leaving the water to the side (waterfall) and sometimes there's weird movement between swimming and gliding when on walls or something like that. So the game partially still tries to glide.
Confirmed for 1.9.3-pre1.

I think It's cool, can we keep it but fix the durability problem?

Hari It's my opinion, I never said you had to agree.

Yes, it looks cool, but it's not just an animation, you actually have some elytra movement bits that have thrown me out of waterfalls multiple times. And if this becomes a feature, then more like those player model mods that add animations for fishing, flying in creative, jumping and so on, not just one single thing.

true, but I still think it looks cool.

can confirm for 16w20a and 16w21a. also @unknown is right; it's not just an amination; you actually don't update into the landing state.
Confirmed for 1.10-pre1

After some more tests, it’s not that the player model does not update; you actually still are gliding (use a firework and you can fly out). And this seems like either intended or an oversight, as the code only checks for a block you stand on in order to land (and you can’t stand on liquids).
Edit: just realized the previous comments, not new info.

This still affects 18w16a.

I remember when this was used for "swimming", but now that we HAVE a swimming animation, this is useless.

Can confirm for 1.13-pre7!
[media]

It's a bug, so it's not meant to have a use

The following is based on a decompiled version of MC 1.12 using mcp940. Please link this in the description of the report.
In the method net.minecraft.entity.EntityLivingBase.isElytraFlying() are no checks for if the player is in water or lava.
public boolean isElytraFlying()
{
return this.getFlag(7);
}
To fix this you'll need to add a check or the player is in water or lava, as seen here;
public boolean isElytraFlying()
{
return isInWater() || isInLava() ? false : this.getFlag(7);
}
This should stop every property from an elytra when in water or lava.
Hope this helps 🙂

Confirmed for 1.13.2.
When you enter a body of water from an elytra flight, you will stay in some kind of semi-flight mode during swimming. You will be swimming, making swimming motions etc., but also flying. This is made apparent by two things;
-Your elytra wings remain in a spread out position as if you were flying, which would not be the case if you were swimming normally;
-When you reach the surface of the water, you will not be able to return to an upright position, as you would when swimming normally. You will stay horizontal and only unequiping the elytra or touching land will make you go upright.
If you touch land at any point, this semi-flight mode will finally be cancelled and everything returns to normal. But if you're just trying to surface from the water in the middle of an ocean, you will be forced in a horizontal position because there wouldn't be any land to cancel flight on.

One potential alternate solution to this (other than changing the above mentioned code), could be to cancel flight when swimming upwards using the Space key. This would make sense, because that key is used for swimming only, and when trying to swim, you shouldn't be flying anymore.

Confirmed for 18w48a

Confirmed for 18w48b

Confirmed for 18w49a

Confirmed for 18w50a

Confirmed for 19w02a

In my opinion this should be a feature. Gliding while in water with a Riptide 3 trident, is really useful. Without gliding before taking off, it is a lot harder.
I think this is a useful bug/feature.

Just not being able to get back in upright position eventhough you're not flying anymore, that is an unintended side effect of this.

Can confirm that you cannot swim normally after gliding into water in 1.14.4. Very annoying.

Understandably it was marked as intended that the gliding continues when you get into water (so you cabn glide through). However the player model getting stuck in this position is now tracked in MC-162919

Tencryn
Cannot confirm in 20w17a. You swim at the same speed with and without elytra activated in my testings.

That is not what this report is about. And this report was also resolved as WAI, with the new swimming animation being the same as the gliding animation.

Leaving/surfacing is not like swimming because that is usually walking but is elytra, so it makes this weird fast bobbing and gliding action.