When you run a command that teleports the same boat to the same location every tick, the boat falls down for about 1 second before it comes back to where it should be.

What I expected to happen was...:
The boat not moving at all, or maybe just a tiny bit.

What actually happened was...:
The boat seems to sink.

Steps to Reproduce:
1. Place a repeating command block and set it to always active.
2. Pick some coordinates near the commandblock, with empty space in a 10x10x10 cube around these coordinates.
3. Place down a boat somewhere.
4. Set the command in the command block to

tp @e[type=Boat] x y z

and replace x y z by the coordinates you choosed previously.
5. Look at the boat going up and down.