Hobie's can be rented from Sailsports and they will deliver to your hotel. You can find them at http://www.sailsportsbelize.com/

As for the 3-night minimum you'll find that at many hotels during peak periods and high season. It's easier and less risk to keep a week open than to try and sell the remaining 4 days. It's not just on the island, when I used to build custom itineraries for Belize Expeditions I often ran into that problem across the country during peak periods.