Having just returned from a Liveaboard a couple weeks ago, I highly recommend:
- Cactus Juice
http://www.cactusjuicetm.com/ProductDetail.php?id=4
It works great for soothing sunburn, insect bites, jellyfish stings, rash, etc. Kind of like several products in one bottle. My dad was stung by several box jellyfish on one of our dives and someone on board had this. One application greatly reduced the pain, and after the second application he couldn't feel the sting anymore. Much nicer than having to bring something for Jellyfish, something for insects, and something for sunburn. I won't go on another trip without it, it was that good.
- Earplugs, definitely. I used these:
http://earplugstore.stores.yahoo.net/silnatrubear.html They are fantastic. The boats are small and sound travels well. It is really helpful to be able to take a nap and/or sleep at night without being woken by people who have very different sleep schedules than yours.
- Lots of sunscreen and a bucket hat, esp. if you burn easy like me. It is nice to be able to bring your own shade with you if you need it, esp for the neck and ears.
- Your own small first aid kit. We had so many little cuts and scraps that the boat almost ran out of band-aids.
- Other things I was glad I brought: save-a-dive kit, sink the stink (or other type of dive deodorizer). Next time I will take like a travel size bottle of woolite or something like it, so I can wash something in the sink if I had to.
Have fun,
Jason