If you're going to be sitting around in a wet bathing suit for a week or so, I'd take Diflucan tablets. But maybe that's just me. (Sorry if this is TMI for the menfolk.)
I also always take a run of mild antibiotics (it's soooo easy to catch the cold of whoever's on the boat, given the confined spaces and shared eating utensils...don't want that turning into bronchitis)...earplugs, since I'm a light sleeper and may be bunking next to the head; an IPod (useful for backing up photos, also music); about as many 1- and 2-gallon Ziplocs as I can carry (keeps stuff dry, also you can see it); some plastic alligator clips for hanging stuff from the rigging to dry; my own insulated mug, since I'm a wimp about drinking hot beverage; spray conditioner (I like the John Frieda anti-frizz stuff) for putting on right after leaving the water, so the salt water doesn't make my head into a rat's nest; a couple bandannas (useful for all kinds of stuff); a fistful of change in various nationalities (good for screwing stuff together in my camera housing); and last but not least several packages of Wet Ones (usually the antibacterial kind). They're good for cleaning your hands and getting the salt off your face.
I also usually go to the Goodwill before the trip and buy t-shirts and a white long sleeve shirt for a couple bucks. That way I can spill stuff all over them and it won't matter. Believe me, everyone will look awful and nobody will even notice, cause you'll be having so much fun. :10:
On towels: If you buy big nice beach towels, they will not dry. Remember if you're on a liveaboard, you'll be relying on Mother Nature to dry your laundry. You can buy cheapo beach towels for $3 or so at Walgreens that are good sized and dry quickly. Since they're inexpensive you can leave 'em behind if you buy a bunch of souvenirs you need to take home.
Oh, and from a ways back? I'm finally going to make Divemaster this weekend, and I can't imagine anyone getting angry if you take a small waterproof camera on a training dive. Just don't take a big, huge heavy SLR housing or anything, the kind with huge lights and stuff!
Have fun and good luck!!!!!!