My list:
1) BUDDY CHECK - to me, this is the most overlooked diving skill that there is... B W R A F (Bangkok Women Rarely Are Female - it's a Thai thing, maybe you had to be trained there)...
Let's review why I say this with a few simple examples from my OW training and subsequent dives with a group that changed a little daily during my four days there...
B - BCD/Buoyancy - the idea is to check inflation and deflation - but people don't and guess what somehow people end up in the water without their air on... HOW?
W - weights - this is supposed to be both a check to see if they buckle is reachable in a dump situation.. but if you are diving with a buddy, shouldn't THEY check to see if they can find your buckle? I weight 300 pounds +, I have a little ROLL that hangs a little over my belt... it prevents me from easily releasing the buckle, so for sure it would my buddy if they don't CHECK it... the other thing is that for this reason, I rotate my belt to have the buckle right under my Alt air source... easier to reach, and the buckle releases like a charm - but again, simply for the sake of common sense... if you don't check... invariably, someone ends up in the water WITHOUT their weight belt or integrated weights in place.
R - Releases - this also seem kind of repetitious or stupid, but with all the gadgets that divers attach to themselves these days, it makes sense to check which is which, where a knife might be, where an alt light, etc... more importantly, someone wont enter the water with just their waist band fixed and the waist and chest releases still undone... want to see panic in the eyes of your buddy? wait until they jump in and find out they are going to the bottom with just a belt of lead on their hips while their BCD and tank are floating above them... luckily, the DM caught it first...
A - AIR - already pointed out, but is it on? you'd be amazed how many times people forget, and they put the reg in their mouth and roll or stride without a second thought, and also no air.... is there enough air for your dive in the tank? or did it accidentally get passed over as the DMs and boat bunnies were doing tank refills.... the other part of this is WHAT KIND OF AIR!!! If you are doing a deep dive, is your dive plan for Nitrox? Did you prep a nitrox tank? is that what is on your back? Did you test it? Did you change your dive computer over for the mixture %? In a buddy situation, you are going to potentially use your buddies alt air source in an emergency... wouldn't you like to know it works? Did you test it? Did they test yours and know if it's tucked up under a BC strap, or on a magnetic release? or in the pocket... lots to consider over just checking it's turned on....
F - Final Check - are you both feeling all right? are you both happy with your hand signals? And ready to dive?
2) Mask off - mask on - learn to lose your own mask, learn to pull your backup (do you have a backup?) practice clearing, practice swimming and ascending without it..
3) Lose the 2nd stage - practice at a nice sandy spot on the bottom, pretend YOU are the instructor, demonstrate it, and then have your buddy do it, get used to finding your regulator
4) Dead/flooded flashlight - do you know where your spare is? can you remain calm while you get it lit up? what happens when your backup fails? Do you carry a high-vis chem stick in your BCD pocket? which one? Can you find it and activate it in the dark, underwater?
5) Knife - where is it? If you ask me, I will say.. WHICH ONE?
Anyway, I have a whole 20 dives to my name... but the aviator and my military training have taught me never to take anything for granted, and checklists are important, because they create good habits...
When I was flying $6MM helicopters, I could easily go thru the startup procedure from memory, but there is a reason they have someone READ it to you.. mistakes happen when people are allowed to control CHANCE...
As a note, for the buddy check above, during those four days with the dive boat we had 22 different divers with a core of 8 who dove every day... I personally witnessed each of the failures (some more than once) mentioned... the scariest of all was that gal who would have potentially gone to the bottom as she slipped out of her BCD...
Pay attention to the details is my motto, the rest, I can practice in the water...