Take out the lifejacket, there should be enough on the boat for everybody, and if there's not, don't put to sea in it, coz wot else have they overlooked.
That leaves the thong. Add a pair of shorts and a tee-shirt for formal wear. Sailing shoes, and jesus boots for shore expeditions. Sailing gloves if you suffer from tender parts. Something warmish for hanging around in air--conned airports. Credit card. Sun hat, shades on a string, and factor 30 and you're all set.
Right then, packing continues. Following advice I've added a hat, shades and some sun cream. Also a twin pack of Wrigleys - so much more convenient than a toothbrush.
Still taking my own lifejacket as it's got all the goodies attached that one's on board may not have.
Also considering something warm for night watches - is a barbeque practical?