Simply standing on top of a boat will cause you to get "sort of stuck", and you'll glitch.
Stuttering movement when walking on top of boats on water; same applies to some extent to moving on top of a shulker
Glitched collision with shulkers and boats
player glitch on boat
Glitch when jumping on boats
Dismounting a boat in water while facing a 2-high wall causes you to get stuck inside of the boat, unable to move
When exiting a boat it will put you on top of the boat