Here is the link we used to make our reservations through Island Marketing.
From what I have gleaned in researching our upcoming trip, they limit the number of reservations they make per day too 100 or less. (most reports, seem to be more like 20-30 people tops, except during spring break.)
The only other people who are allowed into Nachi Cocum, other than the Island Marketing reservations, is a few people from a nearby hotel, who get shuttled over from time to time.
Some of the other beaches, (I think it was Playa Uvas) allow some of the party boat excursions to stop in, during the cruise, which leads to a huge influx of drunks interupting your day.
Paradise Beach seems to be a nice, family alternative, with inflatable icebergs, etc... for the kiddies. (plus only a $9 fee to use their toys)
But we are looking for as kiddie free day as possible, just to RELAX by the pool (which Paradise Beach, doesn't have)or the ocean and chill out.
The $49 AI fee seems to deter many families from bookng Nachi Cocum, whihc is fine by us.