When viewing a player break their boat they will seem to teleport back to where they entered their boat, the player can then be viewed in this "old" location moving around until they touch solid land, after touching solid land they will be seen to teleport to their actual location.
Note: This glitch does not affect the person breaking their boat, it can only be noticed by people observing the player break their boat.
How to replicate:
1. Have a friend on a seperate account enter a boat and travel off into the ocean.
2. Have your friend break their boat while a distance away from you.
Video of the glitch:
Tested on a vanilla server (no plugins)