Upon reaching Harbour Island, most visitors don't need transportation. They can walk to where they're going, rent bicycles (check your equipment carefully before renting -- some bikes rented to tourists are way past their prime), or putt-putt around the island on electric golf carts. Most hotels offer these for rent or at least will make arrangements for a cart or bicycle; usually, they'll be delivered directly to your lodging.
Michael's Cycles, on Colebrook Street (tel. 242/333-2384), is the best place to go if you want some mobility other than your own two feet. The shop is open daily from 8am to 6pm. Bikes rent for $12 per day, two-seater motorbikes are $30 per day, and four-seater golf carts go for $48 per day. You can also rent kayaks for $40 per day, paddleboats for $40 per day (or $10 per hour), and jet skis for $85 per hour.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.