No you’re not seeing things. Its nude boggie boarding! Those of you who have been to Sitges before are aware of how liberal the sunbathing scene is in Spain. However for our “Sitges Virgins”, there are several Nudist Beaches in Sitges.
The two most popular are Balmins (not gay, but gay friendly) and Playa del Meurte. Check out Gay Sitges Guide’s Beaches Section for further information on the beaches of Sitges.