We had an interesting experience yesterday. Upon arriving at a popular beach and anchor area, we dropped anchor about 150' off the beach. We then swam to a friends boat that was anchored a few feet away. We had a great time socializing from a distance for several hours. About the time we decided to pack up and head back to our boat, the Admiral screamed, there is someone on our swim platform. I looked over and observed 3 guys hanging out on our swim platform, one was doing backflips and what not. We expedited our trip back to the boat. Up arrival, all 3 were still there, just sitting on the swim platform, and did not leave until we told them to depart. I don't think they ever entered the cockpit or had bad intentions, they just s aw an opportunity to use the swim platform for recreational uses. Surely they must have known that the owners were nearby? Has anyone else had a similar experience?