For the most part, I still try to pack like I'm going to live on a 34-ft salmon troller. I've seen a lot of burly tugboaters casually wrestling with enormous rolling luggage or haphazardly-filled garbage sacks. If you're ladyfolk, though, your luggage will somehow become a Symbol of Gender and boatbros may give you shit if it has wheels or colors or seems, ahem, "delicate." You can plan to minimize this, or you can go get a Bedazzler and cover your luggage with an enormous middle finger made of rhinestones. I support you regardless.
My bag is a canvas duffel from Goodwill. I sewed a skull and crossbones on it. For morale.
The luxury of on-board laundry means I can pack clothing for 5 days and plan for two wash cycles during the 15 day tour.
Summer is light, ripstop pans with many pockets; crewneck tshirts with some sleeve (I buy men's undershirts in black: they're fitted without being form-fitting, don't advertise my armpits, and don't show engine grease), thin wool socks and a hoodie or windbreaker. Winter is all that plus longjohns and a wool sweater, thick wool socks, and a safety-yellow wool hat knitted by a dear friend. And a backup hat. And more socks.
Bring sleepwear appropriate for emergency muster and/or bathroom trips through the galley.
Always bring rain gear. I personally don't like the Tactical Breathable Gore-tex variety. The material’s magical pores fill up with grime, I don't have the time or space to be re-treating it with whatever fancy spray it wants, and it gets ripped open regularly by the many sharp corners on deck. I like the old PVC stuff that is basically two squares with arms and legs and a hood slapped on. Viking Journeyman Oil Resistant costs $30 or so, and has lasted me years. This is also good stuff for commercial fishing (just chop the sleeves off at 3/4 so they don't wick fish blood up to your elbows).
A Kindle and an mp3 player. CHARGERS. Toiletries, always to include a Diva cup, baby wipes for removing engine oil between showers, and heavy-duty lotion. Travel-size, screw-top bottles for all liquids. Two tennis balls and a giant rubber band for DIY physical therapy. Stick-on disposable heating pads are pretty great, if you get cramps.
A pair of leather Keen shoes and a pair of steel-toe rubber boots (Xtratufs are flexible and have good traction, and make you look Super Legit on the West Coast. Seriously, I think I got my job in part because I wore these to the interview).
A chart book, so I know where the hell we are when there's no phone reception.
Extra gloves. A headlamp and flashlight. Locking-blade pocket knife. Victorinox serrated knife for emergencies, strapped to my life vest. Life vest pockets also contain a tiny winch repair kit, electrical tape, cheap plastic safety glasses, and a continuity tester. Basically anything I would need on the barge and not want to spend 5 minutes fetching.
Basic meds, if the boat doesn't have them. Melatonin, if that's your jam. Good chocolate and your favorite tea.
And since I don't work on tiny boats anymore, and the staterooms are very soundproof: a banjo.