I have been on a few cruises over the years. I’ve always worn my swim thong on the ship and at the beaches usually I am set up on the top overlooking the pool on the ship and have people next to me and walking by. While I have heard a few rude comment from some ugly Americans, I’ve never had a problem and have had more complements that anything else. You will see women wearing thong bikinis. You’ll be lucky to see any man in something smaller than swim shorts. At the beaches it has never been a problem wearing a thong (private island or otherwise).