TOWELS.
Most of their boats have driers (like the ones in laundry rooms), but on our first trip with them it was broken. Places to hang wet towels up to dry were at a premium, so it was really handry that we'd brought a few extras!
Bring a water bottle to refill on the boat so you don't have to leave your bunk in the middle of the night for a drink.
I have a collection of random toiletries and medicine that I always, always bring now, because whatever I might have left behind I'll wish I had. It usually contains cold meds, seasickness pills, earplugs, a small thing of lotion, foot powder (for itchy feet after sitting in saltwatery booties all day), tissues, spare contact lenses, immodium, etc.
Definitely some warm-fuzzy clothes to wear between dives and at night, though hopefully it'll be nice and warm while you're here. We usually dive with them in November, and I can't stress enough the importance of something dry and warm while you're waiting between dives!
Have a great time; you'll love the Vision! When we were on it, the boat was totally packed and then some (a few non-divers along for the ride; at least one couple wound up sharing a single bunk) - and it was still the roomiest boat I've been on in SoCal.
