1. Overhead stoage over the diver benches for small breakable stuff (a tight knit cargo net hammock type works good). Agree +1
2. At least 10 square feet of space for each diver to dress out for a dive. 3x3 is not asking much, agree.
3. deep benches that excess gear or bags can be stored under, or designated dive bag storage. I have been on one boat that had platforms on the gunwhles that hung out over the side with bungie straps to lay dive bags on. It saved deck space, but was a little percarious if you were trying to stoe something. Very important that you are comfortable with your gear storage which is always a problem
4. A head accessable from the wet (main) deck with enough space for a diver to pull their dry suit down and use the head. Amen to that, don't like bouncing around and getting pee spray in my drysuit
5. Snack area that is accessable on the wet deck so divers are not dripping water all over the dry (i.e. state room) areas. Also a waist high table (2' X 3' is big enough) to sign log books and paperwork on is good. Agree but more of a wish list for me
6. Depending on the number of divers chartered, a Rubbermaid medium to large storage tub filled with fresh water makes a good camera rinse bucket. Have a smaller bucket near the dive exit with either fresh or salt water for divers to rinse mask before jumping off the boat. Clearly identify both with signs so there is no mistaking the camera rinse for a mask rinse. Just about standard on all boats
7. A designated location off the deck for cameras and other wet delicate gear that is away from the snack area. Again, wish list only for me
8. The 45 gal. drum would make a better post-trip equipment rinse tub. I wouldn't fill it until headding back to port so divers can dip their dive kit before returning to dock. Agree, post trip only, use that space for something else
9. Dedicated wet suit hanging locations are nice. There is a boat in Hawaii that has a rail down the center line that wet suits are laid over to drain and the additional scuba cylinders are stored underneath in a row of two along the center line of the boat. Wish list again, could live without it
10. Lots of hand holds, like 1 1/2" railings where ever divers need to move about. Divers in their gear are top heavy and need places to grab to stay up right. BIG yes for safety. Agree!
11. Most important, the hand rails on the dive ladder(s) must be wide enough for a fully rigged diver to get up. 2 feet and 6 inches from rail to rail is probably good. And divers should be able to reach the railings from in the water as they get ready to climb, and the rails or some type of hand hold should be accessable all the way up to the deck. The bottom rung of the exit ladder should be around 4 or 5 feet under water when the ladder is deployed. It sould also be secured so that it does not bounch up and down when the boat moves with teh swell and waves. Pich points where the divers need to clib up are bad and I have seen a number of poorly designed ladders that bounch up and down with exposed hinges to pinch fingers. Absolutely. A ladder capable of being used with fins is another potential option.
Without looking at your vessel I don't have any other comments. If it has a typical tug boat gunwhale that runs around the back deck you might want to think about steps up to a small landing on the gunwhale and then steps down to the water or swim step where the ladder(s) will top out at.