If you are just visiting Water Island, there is no transportation available to rent, however, the only place you will probably want to visit is Honeymoon beach which is about a 3 minute walk from the ferry dock. All private house short-term rentals come with a vehicle (usually a golf-cart) for transportation around the island during your stay. Considering this is a residential island with virtually no commerce, it is likely to stay this way, and a reason there is virtually no crime or traffic accidents. An alternate mode of travel to the island is by boat which could be anchored in Honeymoon (Druif) Bay.
Favorite thing: It is the most popular at Water Island. I have to confess though, that I was really disappointed once we got there and saw dog's poo at the sand, not too far from the water. However it was a pretty beach, not good for snorkeling, though.