Most of the resorts on the beach in Tulum will have access to a secluded spot where you can skinny dip.
Xamas Dos and
Nueva Vida de Ramiro both are know for nude beach use but I'm not too sure about how strolling naked from your room to the beach would be accepted. It would depend upon the guests staying at the time.
Mak Nuk in Tulum is much less rustic and more nude friendly but not on the beach. The condos are quite modern and comfortable. The pool at Mak Nuk is one of the best we have seen in the area. Each first floor suite has a private entrance with a small channel leading to the main pool. The second floor suites have private hot tubs and the top floor is equipped with individual plunge pools. They frequently run a shuttle to their (as of now) unimproved section of beach to enjoy the Caribbean au natural.
Also in that area is
Hidden Beach Resort . A very high end, all inclusive hotel that is all nude all the time. They have really great service, wonderful activites, a fantastic pool but the beach is puny and rocky. They are a little expensive too.