spare mask, especially if you use prescription lenses (and mask defog)
get NITROX certified, and use it! (and bring your C-cards)/DAN insurance cards.
stay hydrated, I bring a small plastic water squirt bottle and keep it out on deck in the dive bag, it's great for getting a swallow of water whenever I want, also use it to rinse off mask defog as I'd highly recommend you NOT use the communal mask bucket!
buy some dive socks to wear under booties, you will be doing a LOT of finning (25 - 30 hrs underwater over a week @ 4-5 dives daily) and blisters on those feet will really suck!
learn how to use your computer, bring a back up computer (preferably the same/similiar unit...and bring a paper copy of the owner's manual for you to reference...many divers who wait longish periods between dive trips sometimes forget all the details about how their computers work.
(about computers, I hope you're not diving a Sunnto computer, they are ultra conservative and you will get a lot less bottom time than other divers not using a Sunnto computer.)
don't forget the basic emergency signaling devices (decent sized safety sausage/air horn/strobe)
contact the boat and find out what gear they have on board to rent/loan on an emergency basis if your luggage is delayed, which might help you figure out what to bring on your carry on versus checked luggage. Most divers bring at least their reg/computers on their carry on.
make sure you're bring good fins you really like, or it's gonna be a really LONG week!
I keep and update my 'gear weight list' meaning I weigh my major gear items and put that on my inventory list, it helps me plan what gear/how much gear I can bring and how best to distribute it across however many bags and lbs-per-bag I'm allowed.
at least a pair of thin reef gloves is a good idea, for holding onto down lines in a current.
don't underestimate your thermal protection needs....you will be underwater a LOT, which drains heat and will drop your core temp over time, err on the conservative side, if you can't decide to go with 3 mm or 5 mm suit, go with 5 mm suit, for example...and wear a thin hood or beenie at least as something like 40% of heat loss is from the head/neck areas....this is one of the most common newbie diver or liveaboard diver mistakes, and if you calculate how much $ each dive is costing you, do you really want to have to sit out any dives because you're cold ?