Agusta Koala, Bell L-4 , Bell 407, all spring to mind.
As the outlying islands are 9-12 miles from the center island....even a good JetRanger would work despite being limited to 3-4 passengers.
If you use the helicopter to shuttle to the island....it will be hard to compete with an airplane for the mainland/island shuttle....as airplanes are a lot cheaper to operate.
Any bigger then the thing gets too expensive.
I would go Turbine powered....no matter what!