We stayed at the Iberostar Paraiso Beach/DelMar in August. At that time, there was about a 1-hour wait for the Hobie cats and kayaks. We didn't windsurf, so I don't know about that, but we usually saw sails sitting in the rack. The kayak sailing time was about 45 minutes at a time, but I think you could go longer if you went early or late in the day.
You might find it a little busier in May than we did in August. High season in the Caribbean is usually from late December through April, then gradually tapering off. (However, the Iberostar is very popular with Europeans, and they often vacation in the summer.) I don't think you'll have to wait too long for equipment. They seemed to have plenty.
We did snorkel a bit at the Paraiso beach, but the visibility wasn't great. Rocks weren't a problem, since there were enough large sandy sections, but the water was pretty rough. Made for great body-surfing, but not the best snorkeling.
There are lots of good snorkeling spots in the PDC area. Playa Maroma is close to Paraiso beach, and we loved the boat snorkeling trip there. We saw incredible corals and so many fish! You can see plenty if you are willing to take a short trip off the resort.
We really enjoyed the Iberostar Paraiso Beach/DelMar. I hope you have as great a time there as we did!