I think you should have the ability to build some kind of steamboat. A little bigger than the stock Boat, and with a Furnace in the back, with two paddlewheels on the side, as well as a Chest and an extra slot for a person. In exchange for all that it'd consume coal while moving.
Part 3 of Ocean Improvements will be about transportation, so it will definitely include bigger boats. However, they won't be steamboats.