The problem:

  While sprint-swimming, the player may jitter and be unable to move when swimming close to a ceiling. This may be easily encountered when exploring Ocean Monuments.

How to replicate:

• Find or create a ceiling of blocks that are submerged underwater. 
• Under water, hold Sprint with ALT (double-tapping W to sprint won't produce the bug)
• Continue to hold ALT while punching an entity once.
• Now, while still holding ALT, swim towards the ceiling. The player should begin to jitter and be unable to move as soon as they are 1 block from the ceiling.

What's Happening:

  I've attached a video demonstrating the issue. As you can see at about 9 seconds into the video, the player holds ALT to sprint-swim and punches an entity underwater. The player briefly stops and starts sprint-swimming again even though they are still holding ALT. The game acts as if the player is still in a standing hitbox (2 blocks tall) when the player's hitbox is actually swimming (1 block tall). Thus the game doesn't agree with the player's position, warping them back to where it thinks they should be.