I was thinking and wondering and had an idea. Speed boats! The way they would be made is like the powered minecart but the method would be:
Build two boats
Build a boat engine which would be furnace over a boat
Put the two boats next to each other on the crafting table And there you have a speedboat!
If this would be to easy then you could maybe have to create a Propeller which would be on the crafting table with a diamon in the center, a stick in each corner and red stone in the middle edges. This would be put under the boat on the crafting table still with the furnance powered boat.
Taking this further a chest could be replaced by the furnace creating a much slower boat but one you can transport goods with.
I know the game doesn't need this but i could see it as a fun way to get about and to create sea based adventure maps!