I'm not sure if this is a bug or if it works as intended.
If you fly into the water using elytra, you are able to use rockets under the water for a boost.
This doesn't work if you jump into the water without flying.
In third person view, the player remains in the "flying" state even when in water if they fly directly into water.

How to reproduce: fly into the water using elytra, and then use a rocket underwater.

I have a video of the bug, but it was too big for the max file size limit, so I put it on google drive.
https://drive.google.com/file/d/1tdg6GUP3cwNclVP4CPG8WybB4zNBijTJ/view?usp=sharing