Reunion, as you said, is a very large resort. One part of Reunion is a Wyndham timeshare property. All of those are nice 3 bedroom condo-style units with 3 baths. You can rent those directly from Wyndham, or from a Wyndham timeshare owner. Renting from an owner will be MUCH cheaper.
All Wyndham guests enjoy full waterpark access for the length of their stay. Non-Wyndham guests may get one day, or you may have to purchase a ticket to the waterpark. I would be sure to get those details clear before you book. You can see the waterpark with a simple Google Images search.
I assume that other types of accommodations would be renting directly from a homeowner -- condo or house.