Depending on the length of the passage, I'd avoid the '2 man' crew on anything longer than 4 or 5 days. The 4 on 4 off watches are on the brutal side and can effect decision making and rough weather will only make matters worse...especially at night. After 10 days, I was headed for the 'unfit line' and although still functional, I was very tired. After the twelfth day, I was more than ready to jump ship.
I like the 'watch bag' SVAuspicious suggests. I usually bring enough snacks (beef jerky, trail mix, granola bars and chocolate) for all but I save my favorites for watch snacks...helps on those lonely nights when you can't see anything but the LED's in the cockpit and the night demons are doing there little dance in your head.
EAR PLUGS! Don't leave home without 'em. It's amazing when you get overly tired how noisy a boat can be...vhf (buddyboat), ssb, tankage slosh, that lost can of rust buster bumping up against a half full water tank and of course the Capt's lucky charm hanging from a chain on the bulkhead next to your head that's the size of a trash can lid...which BTW, apparently worked well!
Alarm clock. Nothing more irritating than being late for a watch. I carry one of those Oregon Scientific models that emit a light high frequency "beep" that can be heard thru those soft db 32 ear plugs yet will not disturb other crew. If especially tired, have a wake up call by the on watch a half an hour earily to make a snack and something hot for your watch and the one coming off. Helps in keeping awake during watch and serves well as nap fuel for the one coming off.
Beside the items mentioned by others above, I carry zinc oxide, extra eyeware, extra pair sunglasses, two goofy hats that cover ears and back of neck (an excellent look BTW) and an extra recharge kit for my harnessed PFD. Oh, and any necessary meds plus some extra...even with the aid of ear plugs, the voices can get quite loud!