Its a small motorized boat that seats one or two people. The ones at CBR and YC do not go very fast but I've heard that the ones at Poly & WL are much faster.
My kids rent these every time were at WDW. They are a little motor boat. We rent them at the Poly. They will seat 2 to a boat but the kids always get one for themselves because they go much faster that way. There is a discount for DVC and AP holders.