1- a ladder that has the capacity for a diver in full gear plus ~15%.
2- a method to secure your tanks, one sturdy enough to hold the tanks in place during rough seas.
3 - a few drop lines of varying lengths with snap rings to secure gear/stage bottles/catch bags.
4 - hot water transom shower.
5 - fresh water tank for camera gear if applicable.
6 - oxygen, first aid kit, blankets.
7 - reliable communications, not just a cell phone.
8 - non-skid deck coating.
9 - appropriately sized dive flag and mast.
10 - a proper extendable boat hook and throw-able pfd.
11 - beer.