When you launch a boat into the air, via slimeblock, driving off a cliff, etc., you can move around like you are in the water, though you still fall. In this respect it is sort of like a less powerful elytra.
Is this still an issue in the most recent versions (currently that is 1.10.2, or 16w42a) of Minecraft? If so, please update the affected versions and help us keeping this ticket updated from time to time. If you are the owner/reporter of this ticket, you can modify the affected version(s) yourself.
Probably related to MC-96195. Since you can move a full speed on land with them also.