After living aboard for 7 years, looking back on what we THOUGHT we'd need when we moved here from Michigan in 2002:
*half as many clothes as I initially brought. Everything in the same group of colors so you only need one or two sets of accessories, rather than 3 sets of shoes/socks/belts etc in black, brown, and navy (for example). Almost everything I own now is black, beige, white, or medium blue - everything goes with everything else, making getting dressed easy. Seek out, and pay extra if necessary, for clothes that are wrinkle-free and don't need to be dry cleaned - there are even washable wools now. And until a couple of weeks ago, I had one of *those* kind of dressup desk jobs.
*twice as many tools. Its a boat, after all. Things break. And if you're living aboard, using stuff every day, its going to break more often. I have one friend whose idea of a "toolkit" is a cellphone and a checkbook. For the rest of us (a) sometimes things break when you're in a pretty primitive place; (b) sometimes things break and need to be fixed NOW because water's coming in; (c) money's not infinite.
*a quarter as many kitchen thingeys. A good sharp knife is more reliable than an electric food processor, and takes less space too! Take nothing that serves only one purpose (hand-cranked pasta machine? heart-shaped cake pan?) Our galley has some good-quality stacking pots and pans; the aforementioned good knives, whisks, spatulas; an eggbeater; lemon zester; corkscrew; can opener; Melitta-type coffee filter; soup ladle; and that's about it. Oh yeah, and a grill <*winking at CD*> You're likely to be spending a lot more time socializing, so a few serving trays or break-resistant wineglasses/beer mugs/whatever you drink are in order too.
*you don't really need every gadget they hawk at the boat shows ...
*think space-saving. We put all our CDs on an iPod and all my cookbooks on computer files, for example. I'm thinking about getting a Kindle for the same reason, but there's always book exchanges at marinas.