Big clips of some sort to fasten your skin, wetsuit, swimsuit etc. to the hangers on the top deck. At least twice I helped a crew member in a mad dash to keep garments from blowing overboard when we passed through a squall late at night. 2nd time we lost a couple t-shirts and somebody's skin. I used an old coiled lanyard to secure my swimsuits and t-shirts to the side railings, in the breeze they'd often dry before dinner. A short piece of clothesline might've been better.
Watch the Rorqual slideshow. See the cabin photo with the power outlet behind the pillows at the head of the bed? Might want to bring an extension cord if you have chargers etc.
Lots of sunscreen and a big floppy hat. On the top deck you're the tallest thing around for miles so you get cooked by the sun reflecting off the waves and white boat surfaces.
Other things to know:
Since the mooring on the boat is on the deck high above the Swath - following the mooring line up can still mean a healthy swim back to the boat on the surface. They often run a line around the boat if that's the case. And because it's a big square box the wind moves the boat pretty good around the mooring. We did one dive where I surfaced, spotted the boat and then descended to swim to it. In the meantime it blew over our heads to about where we'd been - never saw it go over.
Are you doing Cay Sal? If so when you do the Big Hole dive early in the week the sharks will congregate near the boat as you're very close to the site where they do the feed later in the week. We wasted most of our first dive in the murky hole - there was a sand fall on one side that ruined the viz - coming back up to the boat we were escorted by non-aggressive sharks. My avatar is one of them. I was filming in 15-20' of water till I ran out of air. At one point my buddy counted 12-15 of them circling me. On the second dive some of us went and sat in the grass on the bottom. The sharks came within about 3-4' of us if we waited patiently. Totally amazing experience. The first jump through them is a leap of faith - they twitch slightly every time a diver splashes nearby.