Get some UPF (ultraviolet protection factor) rated clothing...
it will be a lifesaver and save you from a really nasty sunburn. Columbia, Patagonia, etc., all make it.
Get some good moisture wicking base layer from Patagonia, REI, or someone like that..
. it will keep you far more comfortable and warmer than the cotton stuff.
Pack gloves and boots and some polar fleece to put on under your foulies.
Here's a hint... the water temp off the New England coast is still fairly cold, and the air temps are going to be affected by it. I just finished a Georgetown, Exumas to Marion, Mass delivery, and we were wearing polar fleece and foulies for night watches the last couple of days. Also, remember that a tropical thunderstorm will have rain water temps in the low 40s in many cases, if not colder, since that water is coming from up where the air is below freezing in temp—how do you think hail forms???
Bring a good flashlight that is water resistant or water proof
—preferably an LED one that has both white and red LEDs. A headlamp is a good choice BTW.
Bring a good hat and sunglasses.
The glare off the water is a killer. Polarized lenses are nice but can interfere with reading the instruments if they're polarized at the wrong angle.
Bring a good PFD.
Be aware that some airlines will not allow you to carry the CO2 cartridge...others will. Check with your airlines directly. You should also have a harness (integrated into the PFD is better) and tether, as well as whistle and strobe attached to the PFD. Yes, some boats will provide this gear, but I'd rather trust my own gear than what someone is providing...
Pack a good water bottle,
so you can have something to drink when you go on watch.
Pack a spare set of glasses if you wear prescription glasses. Pack spare medication if you take any.
This stuff should be in your carry-on luggage, where you can guarantee it won't get lost more easily.
Carry a digital camera.
Always good to have photos of a trip like this. A good one, if you don't have one, is the Olympus Stylus Tough 8000 series.. waterproof, shockproof, decent image quality, and does video as well as stills.
Carry a good rigging knife.
You'll have to check a bag if you do this, but you'll probably have to check a bag for your PFD CO2 cartridge anyways. I like the Boye's Cobalt Carbide rigging knives. They are sharp, keep a good edge, never rust, have a titanium marlinspike/shackle key, and can be opened with one hand. I had to dive on a crab pot line on my last delivery...so a good knife is important.
Have fun... fair winds...