Herring Cove beach is your best bet. It is just outside of town and best reached by bike (rentals are available in town). Before you reach the main parking lot you will see the trailhead with wood fencing and many bikes locked up. Hike across the marsh, usually there is a steady stream of men to follow. Once you reach the beach, go left and it gets friskier the further you go. Lots of speedos things and naked guys, even some public sex. To get there you do cross a tidal stream of water 1-2 feet deep.
I'll be there this weekend and love it there. Also check out Full Kit and Body Body for underwear and swimwear. Full Kit has fetish gear and supplies. Have fun!