Minecart Boat

This item would be placeable on rails or water, and moves as quickly as a boat on both. It can hold up to 2 small mobs or a large and a small mob. Two large mobs don't fit, and things are considered "large mobs" by the happy ghast standards. Players are also considered large here, to prevent them from having two people in a functional minecart. Special rails crafted with gold and blaze rods can be used to accelerate it to 32 blocks per second. These new rails do not work on the normal minecarts. 

There are a few ways I see the crafting recipe: A minecart and a boat together in the crafting table, 5 planks around a minecart, or 5 iron ingots around a boat.