Best place is probably at any of the resorts on the Seven Seas lagoon (GF, Poly, CR and WL) because there is such a large water area to drive around in. However, one must be 16 to rent sea raycers there.
At the Epcot resort area, they can be rented at the YC marina and one must only be 5' tall and 12 years old, but the water area is smaller (only the lagoon there and the waterway down to DHS).
We did this at the YC marina on our last trip (3 weeks ago), but I can't remember the exact cost. It was around $95 for two (2) sea raycers for one hour.