What my friend sees:
She gets into a boat and sails away from me, then dismounts at her destination. All seems normal here.

What I see:
She stands perfectly still (where she was standing when she first clicked the boat) while the boat sails away on its own, empty.
When she dismounts the boat at her destination, her player model suddenly flies at extreme speed to her new (actual) location beside the boat.

This also happens with mine carts and horses. I will upload a video of this happening later today.