I will echo TSandM's compliment, that it is great you are already thinking about this. You are getting a lot of very good feedback on the things that will help you become a competent diver (not just an underwater tourist).can you all give your opinions on what are some of the most valuable skills for beginners to master so that they're second nature to us when we need to use them? . . . Right now we figure the most important skills to practice besides "don't die" and buoyancy are:
1. Replacing mask that gets knocked off and clearing
2. Sharing air
3. Randomly asking each other how much air we have (and hopefully knowing without looking)
I guess I'm trying to think of "oh ****" moments that are bound to happen eventually
And, virtually everyone has mentioned buoyancy / trim / weight distribution so I don't need to add to that - it is a given! As several have mentioned, I have never been involved in, or even seen, seen a true OOA situation, although practicing for one is a regular part of my personal diving routine, and a point of emphasis in my teaching, beginning with OW and continuing through a number of specialty courses. Being proficient in that skill materially adds to your confidence as a diver.
My one additional / supplementary point of emphasis: practice no mask breathing and no mask swimming, A LOT. The "oh ****" moments that I have witnessed have almost exclusively involved divers getting some water in their mask, getting some water up their nose, and bolting (or attempting to bolt) to the surface. And, I have seen that too many times. If you can get to the point that your breathing control (which several have mentioned primarily in the context of buoyancy control) is so 'spot on' that you feel as completely comfortable swimming underwater (while breathing from a regulator) without a mask, as you do with a mask, you will minimize the likelihood of injury associated with the 'bolt'. Touching the bottom won't kill you, or even hurt you (although your feelings might be hurt by the piercing glares during the dive, and blunt comments after the dive, of other divers). But, bolting to the surface from 50 ft because you snort water up your nose, possibly even spitting your reg out during that bolt - both of which I have seen more than once - can hurt you. I would never publicly suggest that I might in any way violate, or substantively augment / amend, any training agency standards. But, I have a friend, let's just say he is an instructor a lot like me, who I have been told begins every OW course Confined Water session after the first one with a no mask swim for the students. And, I have also been told this same instructor has OW students perform a mask remove / replace / clear on each of their 4 OW dives, not just on one.