1. Ditto freshwater shower on the deck (for rinsing 1st/2nd stage on egress, and wetsuits later). Also useful for a bit of wet to help slick into a suit after the first day.
2. Wetsuit hanging somewhere it's secure (hangars won't fall or blow off), will catch the wind, but NOT be over the seating for gearing up. Nothing worse than the 'car wash' feeling of being between cold, dripping wetsuits when you're trying to get your own on! Also wetsuit and towel hanging should somehow be separated, but this isn't always going to be possible or obeyed.
3. Easily accessible head just off dive deck for those last-minute potty runs (for boats that don't allow neoprene inside)
4. Bootie rack! This could just be a pegboard with angled pegs to slip your booties over, off to one side, perhaps one per 'row' of divers.
5. No-fog dip and mask rinse tank (or saltwater 'fountain' would be fine) right where you hop off
6. Some sort of dive 'locker' for small stuff (spare gloves, weights, etc) that otherwise just gets scattered around the decking or seating area. The 'seat' could be a small locker, vented below so it doesn't hold water.
7. Camera rinse station (bucket or trough) with a 'wet' table or rack to place it after. External 'dry' tables aren't really going to stay that way no matter what the rules (at the least they're still outside in high salt humidity) but without somewhere to put them they end up underfoot or staying in the tanks - one or two big rigs means no one else has room to even dip, aside from small PnS.
8. SHADE. I will sunscreen my bare noggin to keep from getting scorched, but the second that wetsuit's unzipped I'm certainly not about to start slathering up all over. Of course if the shade is optional that's all the better - a bit of sun on cooler days could really be nice. So this is a bit climate dependent I spose.
9. Small maintenance table and toolkit for tank o-rings, computer battery replacement, loose hose to 1st stage connex, etc.
10. Space the tanks out a bit so divers aren't shoulder to shoulder. Maybe a fin loop to one side of the tanks to space them out - so you gear up sans fins, stand, fins are right there at hand height to snag and not 'under' the bench so you brain the poor guy next to you as you bend at the waist and swing around to reach for them.
What should NOT be there:
1. Dive bags should not be just 'crammed' up under the dive seating. Mine is always half full of salt muck after every flower gardens trip from everyone and their brother and their brother's aunt's wetsuit dripping into it the whole time. (The S/V Juliet www.sailjuliet.com) did this nicely in a way: they had everyone 'unpack' dive gear to their stations before they left, and empty bags were folded up and went belowdeck, stored in an 'attic' area so they were neither in your cabin in your way, nor in the way on the dive deck. Probably not a solution that would work in all cases.)
2. Dry table - it won't be for long. Needs to be somehow separate from dive deck for a bit more safety.
3. Low overheads, or anything that can catch feet or ankles easily either.
4. Ideally, any compressor whips shouldn't be running overhead either where loops can hang down and catch you when you walk (stagger) by with a heavy set of doubles on.