How many times have you gotten to the dive site and found that you've forgotten something important. If you're like me, more times than not.
Here's a packing checklist I came across on the "New Jersey Scuba Diver" site, which I have modified somewhat.
mask, fins, snorkel
BC / harness
exposure suit, under-suit
hood, gloves, boots, socks
regulators, gauges, computer(s)
tanks, bands, pony and/or stage bottles
weights, belt, ankle weights
light ( new / charged batteries )
backup light, strobe light
knife, backup knife
wreck reel, upline reel
catch bag, lift bag
safety sausage, whistle
camera, film, flash, etc.
bug stick, pole spear / spear gun
u/w writing slate & pencil
save-a-dive kit, extra batteries
cooler, ice, food & drink
logbook, c-card, dive tables
change of clothes
payment - check / cash / plastic
travel preparations, directions
set alarm clock
In my save-a-dive kit I keep O-rings, tools (spanner wrench, Allen keys, screwdrivers), duct tape, WD-40, etc.