From Halopedia, the Halo wiki
The unnamed UNSC motorboat is a vehicle used by the United Nations Space Command that was cut from Halo: Reach. A functioning, drivable prototype of the boat was created. Technology was developed to allow wakes to be created behind the boat in the water. Some of this technology was used for the jet engine wash of the Jetpack in the retail game. A water based mission likely would have been where the boat would have been featured.