Went on a boat trip from Calis (from the water taxi area) last summer. £10 each including lunch, or, if you have 6 or more people, you can pay £70 and have the boat to yourselves. There were six of us, so we paid the £70. It was a really lovely day out - nice music, lovely food and scenery, lots of swimming, tarzan swings etc (!). Would really recommend it - very relaxing. (Spent a fortune of cold drinks though, so check out the prices first.)