These are not too common and they are quite complex. Usually powered by a thruster and held up by a gyro, they can use a less complex steering device than the car, which could be a block with a servo on it to change the direction while moving forward.
Impossible to make in SP1 due to lack of joints and no Water Plane.
A fake version could be made by messing around with the physics settings, however this wouldn't be like the newer version
Easy to make now that the Water Plane was added and Thrusters to push it forwards and backwards. Magnets can also be used to pull it around but it would be unstable. The Introduction to Gyros also made it easier to stabilize.