It's Hot on Sin Island (1964)

In Miami Beach, Joey and his partner Ray need some quick cash to finish payments on their 62-foot cruiser. So they charter their boat to five young school teachers on a vacation to Paradise Island. Once on Paradise Island, the girls skinny dip and water ski and prepare "for a weenie roast" all in the altogether as Joey and Ray look on like two idiots in heat. Meanwhile, Clara Calvert, the boat's owner, wants Joey to pick her up so she can get the balance of the $12,000 owed her. Worse, two Miami gangsters, on the run from the law and carrying a suitcase full of counterfeit cash, show up thinking they're hiding on a deserted island. While Joey is off getting Clara, the two gangsters land on the island and stumble across the five hot young women.