Moorings charges $35 for a 2nd dinghy. They don't have bigger dinghies. (not to contradict George, but that's what I was told. If your mileage is different let me know because I would have a bone to pick with my booking agent.)
They will arrange logistics of a larger dinghy from Island Time if you pay directly.
http://www.islandtimeltd.com/I think it was $200/day for the week for a 15' center console.
Also, they don't allow anything bigger than the standard dinghy on the davits due to the load.