1. Take two wetsuits. That way one is always hanging up and dry, so you won't always be putting on a clammy, wet, wetsuit.
2. Also, take a light sweater or long-sleeve T-shirt. The AC is usually kept on the chilly side in the berths and saloon and after diving you're core temperature is always a little down.
3. Check with the operator to find out what water temperatures and conditions you might encounter, and pack thermal protection accordingly. Remember that you will probably get colder as the week progresses (your core temp goes down with extended immersion), so throw in an extra vest or hood.
4. A marker float and compact reel is always a good self-rescue addition, especially if you will be diving from smaller skiffs instead of from a moored mother ship.
5. Go for smaller, high intensity LED lights to save weight and space in the gear bag.
6. Chargers and extra batteries are obviously important, but don't go crazy on the spare parts and redundant gear. You'll likely overload your bags and end up paying excess baggage charges, and most all liveaboards stock a full range of spares and loaners.
7. If you are any prescription meds, make a copy of the order form, both for emergency resupply and to satisfy curious customs inspectors.
8. Pack everything in soft-sided bags, as they will be easier to stow in your cabin.
9. And go easy on the clothing. You'll be in and out of the water all day, so a few pairs of shorts and some t-shirts is all the wardrobe you'll need, plus a fleece jacket. Yes, even in the tropics. As mentioned before, multiple days of diving daily will lower your core temperature, making evening breezes seem downright cold, not to mention the fact that many liveaboards keep the air conditioning cranking in the lounge and dining room.
10. Unless the shore side itinerary calls for wilderness hikes, a simple pair of flip flops or Crocs may be all the footwear you need. Many divers end up going barefoot during the day and maybe slipping on some comfy footie socks at night.
11. With the exception of an iPod, leave the entertainment at home. Most boats have a good collection of books and DVDs, and you usually end up spending the majority of your surface intervals napping or reliving your diving adventures with your fellow passengers.
12. Bring an extension cord, because the electrical outlets are not always next to the most comfortable place to sit while reviewing your images on your laptop.
13. If you are a serious photographer, pack a portable hard drive with a viewing screen and plug-ins for usb and flash memory cards. Epson and Jobo make excellent ones. If your laptop dies on the trip, you're still able to download. And if your laptop doesn't die, you've got your images backed up.
Read more in our Ultimate Diver's Guide to Liveaboards.