Some ideas...
Sunscreen. If you use #15 at home, bring #30. Make sure it is waterproof. Put it on in the morning before you put your suit on, that way you cover everything. re-apply ever few hours to the exposed spots that hit the sun the most. Make sure you get your backside well before you go snorkeling, the water reflects and amplifies the sun.Your prescriptions with enough to cover you for your complete trip.Your passport! I forgot mine once, and it was hell getting out and back in the country.CD's: Most boats have a car type Stereo with a CD player. So bring tunes, but remember to not play them too loud at night.Books: Bring a few paperbacks/magazines. I recommend John D. McDonald, Clive Cussler, and Carl Hiaasen. If you don't want to bring them back, give them to Sydney. Cruisers and liveaboards are always looking for something new to read.What not to bring:Anything else! You don't need hair dryers, more shoes, socks, dress clothes. If you have to ask yourself if you need something, the answer is a resounding no!Seriously, the lighter you pack, the happier you will be. Shoes are rarely worn, even ashore to dinner, and almost never on a sailboat.Whenever I go, I buy at least 6 t-shirts, which I wear, and sometimes a pair of shorts. You can rinse off swim suits the one or two times you shower, and just let everything else dry in the sun. Just make sure you tie them down so they do not sail off the boat.