As far as I'm aware, there is no clothing-optional, i.e. full nudity locations in the Cancun Hotel Zone. The two Desire resorts fit your bill, and of those two, for Stacia & my money, we prefer Desire Riviera Maya (DRM) over Pearl. Pearl is slightly more luxurious, but not nearly enough so as to outweigh the fun times and people at DRM. Again, this is our opinion, and folks' experience differs mainly on the other people there when you go to either resort.
True TTR requires a bottom piece, how much of a piece is discretionary. Narrow, sheer and exibitionistic is OK & encouraged. It’s not nude, but it’s not much. Have fun picking yours.
