Flying to close to a seiling with an allowed ange to not take dmage will deal damage (and maybe appear the drowning bubbles). Also swimming after pressing space will make your hitbox so you can't go through a 1 high gap.

Steps to Reproduce: 

Swimming:

1. Start swimming by sprinting in water.
2. Press jump to go up while you are still swimming
3. Attemt to go through a 1x1 gap in water

Flying w/ elytra

1. Make a platform to use as a seiling
2. Take of with elytra and rockets (or any other way)
3. go as close to the seiling as you can (on survival) and use rockets to keep your speed

Observed Results:
Swimming: You wont be able to go in the gap due to your hitbox returning to normal after jump

Flying: Your hitbox propably hasn't changed at all making you suffocate on the seiling.

Expected Results:
Swimming: The jump action shouldn't change your hitbox

Elytra: You should be able to fly under the platform since there is no critical ange for you to take damage. (Java Parity)

Screenshots/Videos attached: Look Below

Notes: Both the examples are about hitboxes so I dont think separate issues are required.