When riding a boat, you will be able to speed up slightly, then you will stop suddenly. You can travel any distance, albeit very slowly due to starting and stopping. Upon exiting the boat, you will teleport back to your location where you entered the boat. Logging out while in a boat will return you to said location while still in the boat.
Seems to be some kind of lag or something between minecraft and the internal server. Behaves just like bad lag on smp.

Applies to both creative and survival.