Eating a chorus fruit while looking down in a boat and holding down the use button makes the server think that the player is in a boat while the client thinks they aren't. Crouching moves the player near the boat as if they had just left it. In multiplayer other players see the player in the boat.

 
Steps to Reproduce:

1. Sit in a boat far from land
2. Look down
3. Hold down right click to eat a chorus fruit
4. Release right click after teleporting out of the boat
5. Move away from the boat
6. Crouch

Observed Results:

Crouching immediately moves the player back to the boat. Blocks can only be placed or broken near the boat. Attempting to enter the boat only plays the use animation. Dropping items drops them at the boat's location. Running "/data get entity @s Pos" gives the location of the player as if they were in the boat.