When you swim in a 1x1 tunnel and you start to swim upwards, the player model will stand up and its head will be "stuck" in the ceiling. However, you get no damage, your vision will not be stuck in the blocks and you can still move perfectly in the tunnel, so it seems you are still in some kind of "swimming state". You will, however, get pushed out of the tunnel when you are close to some kind of "exit" of the tunnel.

Steps to reproduce:

1. Find or build yourself a 1x1 tunnel filled with water.
2. Sprint in front of the tunnel so you begin swimming.
3. Swim into the tunnel.
4. Press space or swim upwards.
5. Press F5 to get into third-person view. Notice how the player model is standing with its head inside of the blocks above.
6. Move closer to an exit to see how you get pushed outwards.