On a live aboard, there are no dive shops. While the crew will most likely have some spare parts, it's best if you bring extra's of mask straps, fin straps, o-rings for your reg, camera, lights, etc., silicone grease, a back up dive computer, a back up mask and an SBM. I can't tell you how many times I have seen folks jump off a boat without an SBM. Pack extra batteries or even better yet, rechargables. Don't forget converters and I bring a surge strip. I'm a plug pig and my usual roomie actually uses it as well. We are both photographers and use lots of batteries. Also, the crew may not be able or certified to fix your reg. Having extras of some things is an ounce of prevention is worth a pound of cure. At the very least you use equipment you might not be comfortable with. In a worst case scenario, it could mean snorkeling rather than diving. Check with your dive shop. They may have used or rental computers that will be much less expensive than a new computer for a back up. And if you can't afford the back up, see if your dive shop will rent you one.
Where folks generally overpack is clothes. Life is very casual and everything does get wet or damp. Depending on the length of the trip, a couple of pairs of shorts, 4 or 5 tee shirts (one long sleeved), a light weight jacket and boat shoes are about all you need. I generally will bring multiple swim suits. I like to get warm and dry after each dive and I can spread the wet suit out on the deck to dry while I'm underwater again. The only time you will need "real" shoes and possibly long pants are for the air trip to the boat and back home again. I leave all my jewelry home. I use my wrist computer as a watch and alarm clock. My contacts come out the first day of the trip and go back in when I head home. I DO pack 2 pairs of glasses and I have seen them accidently go into the drink.
