1) Plan the dive, you might be reasonably good, but plan the dive. Check that total lift is greater than total extra lead; step through the phases, or at least really think through them. When doing some lift bag practice I walked into the surf zone with fins in one hand, 30lb weight belt and empty lift bag over my other arm, mask around neck (you know, for better all around awareness during the surf transition), long hose clipped or hanging and secondary bungeed. All that would have been fine. I'm happy doing my fins no mask, then doing mask then adding scuba. Expect for that extra 30 lb. draped on one arm, giving me zero arms left over. My BC doesn’t have 30 lb. of excess lift. I managed to get one of my regs in, but kept going under the waves, once I managed to help by walking out far enough. Good plan. My buddy lent a hand and we got it sorted out. A sealed bottom lift bag, which we had, has a new fond spot in my heart. I now want to do some masked and then mask less walking negative beach entries to get that whole transition sorted. I had air, the surf was not bad, and I’m fine mask less, just the down up down up was a bit of a new experience. I did not really want to drop the weight belt and lift bag. Plan it out.
2) Do not be afraid to look the fool or speak up. I lost a slate because I did not want folks to think I had. I was missing it after entry and thought I left it at the car. Later, part of the group was exiting at the entry and I did not tell them that if they saw one it was likely mine. Not learning the hard way at all, but not speaking up kills. We are all colleagues, speak up. The habit of not speaking up is a sin far far worse than any minor screw up. If your friends do not realize this, get new friends, or watch them like a hawk.
3) If you dive Nitrox check the mix in all tank at the start. Two boat dives, deepish then shallower, checked the first tank, 32, but waited on the second. Second dive got to 15’ hanging out while everyone got down and thought ‘Hmmm, I wonder what I’m breathing...’ Asked the guide his thoughts on the number via slate. As anything more than the 36 planned and reported would seem odd, figured it would be fine. No issues and not the hard way, but best to know.
4) Turn your air on. Surface swam out in benign conditions, started to descend together and then signaled up. Once I orally inflated my BC, I asked my buddy if they would not mind discreetly turning my air on, so our boss did not realize I was a complete idiot. We had heavy gloves on, I was not as used to reaching my valve. I am now. Falls under complacency from achieving some obviously low level of competence. (Edit: I may have caught this as we started but aborted descent, my recall is fussy, but it was would you mind turning my air on...)
(From AlexL, Yeah, do not rush getting ready and to the point that you are happy that you are ready.)