I've been on Nekton Rorqual and Explorer Venture Turks and Caicos.
From my "friends" at Nekton (love the crew you ex Nekton peeps), I learned to pay for the trip with a credit card. Then, if the company suddenly ups and goes bankrupt, after you've paid in full for your trip, you
might get your money back from the credit card company. However, if you pay with a check, you're FUBAR'd. Also, get trip insurance, and consider gear insurance depending on how expensive your gear is. Carry your bathing suit, reg, dive computer, and camera(s) as carry on. Check the rest of the gear in your luggage.
On the boat - just apply all the kindergarten lessons - share, play, have snacks, stick with your buddy, take naps, laugh, have a good time. Can't let the small things bother you - and don't be an A-hole and ruin it for others around you. When diving with groups of people, be aware - there will be people with more experience, and less experience, better equipment, worse equipment. Be a nice/fun kindergartner - share, be nice, helpful.
Having worked a few jobs that rely on service and tips, plan to tip. Don't be "that guy". Take extra money for T-Shirts. I love my scuba T-Shirts. Take money for shore excursions - hit the LDS and buy more T-Shirts!!!
Ship board living is tight. Make the best of it by showering with soap, and using deodorant. Lay off the perfume/cologne. As a guy, I don't care if women do their hair or makeup - everyone looks "rough" - no problems mon. No matter how few clothes you take, you probably won't wear them all. One boat confiscated shoes - all week - NO shoes (except for shore).
I really like the buy the crew some movies idea - I'm going to do that.
Stuff breaks, and they aren't always able to have it fixed. Get over it. (unless it's the engine(s) - then I'd understand). (missing the damn hot tub all week ....
)
Night dives are cool - invest in a nice (expensive) underwater light/torch.
This year, my big investment so far - is a digital video camera - with a housing. They'll be NewB videos - but they'll be MY NewB videos!!! Take extra critical equipment - batteries/memory cards etc. Get gear's annual service done ahead of time. Triple confirm any gear rentals. Go with Nitrox - either extra time/or dive on air tables with extra safety.
I'm a nerd - so I like to throughly document the dives, what I saw, a map, etc. Helps me when I forget everything 10 minutes later ...
Friend the crew on Facebook - it's cool to keep up with your crew friends (even though they get paid to make you think they're your friends) - I live my "live on a boat and sail around the world all the time" fantasy vicariously through my Facebook crew friends.
Hope this helps,
PC