Suggest you contact insurance carriers for quotes - they seem to change over time.
Generally speaking -
Golf cart insurance is very cheap ($100 or so a year for our clunker). Would assume scooter is similarly inexpensive.
Boat insurance - I think it's around 2% of the value insured.
Boat license fee is around $100 a year.
Annual golf cart fee - inexpensive.
Permits to bring a land-vehicle to the island are not so easy to get - suggest you look for something already here.