If Pete does not work out, try Trevor Hudlin at email@example.com. We just got back from a week on the Mariner. Trevor met us at the dock and gave us a very good tour of the island. Do Dunns River early before it gets crowded. Trevor will then take you wherever you want to go or provide a general island tour (actually there is not a lot to see outside of the falls - mostly 3rd world country stuff) Trevor charged $28 for adults and $18 for kids which includes the ticket price to the falls.
Very nice guy - beware of the "big" wooden statues - you'll understand this comment when you see them.