Because of Timmy Turner's wish that his grandparents never left Ustinkistan, he ended up living in the impoverish country and everything about his life was changed, including Cosmo and Wanda who could now only grant wishes involving turnips. So Timmy wished for a Turnip Time Machine so that he could travel back in time and convince his grandfather Vlad to go to America with his wife Gladys and start up their Yak in the Box restaurant chain like they did before Timmy granted the wish to prevent them from leaving. He managed to do so, but just as they reached the dock last boat ever to America was already at sea. Timmy wished up a Turnip Jetski to transport his grandparents to the boat so they could get to America. After Timmy dropped his grandparents off, the jetski disappeared when the time line reverted to normal.
The jetski is made out of three turnips, which use the leafs at their tops to act as a propeller in the water. There is a spot carved in the large middle turnip for the driver. The jetski was able to catch up to the boat to America and make a jump into a pool on the deck of the ship.