Innocent Taboo (1986)

Two buddies from the same neighborhood grow up and marry a pair of luscious and sexy sisters. Throughout the years the two families grow up side by side, never dabbling in the partner-swapping salaciousness that they all dream of. They get their chance when they're all invited aboard the boss' luxurious yacht. An erotically enticing potion is mysteriously mixed into their drinks, and the couples find themselves free to indulge their swinging desires. The boat becomes a rollicking ride to the extremes of ecstasy, filled with the luscious curves and sexy squeals of the likes of Keisha, Melissa Melendez and Buffy Davis. The trip ends with an amorous deckside orgy that sends the flick into erotic orbit!