This may be obvious to virtually most, but here are the rules I apply to myself, and I tell people that are new, of just advancing in their diving.
Always have a dive plan, and stick to it. Dive computers are a guide tool, but they do not have the ability to reason. Divers have the ability to reason. Dive computers cannot get CNS - divers can.
Know how to read, and use an RDP Table. If you do not know how to read an RDP Table, learn (YouTube), or you should not be diving. If, for example, you do multiple dives on holiday you are putting your life in the hands of your Dive Master. They are humans, and humans make mistakes. You do not want to fall through the cracks (change in dive times, or moved to a different group).
If you are diving Self Reliant, or Solo (and there is a difference depending on cert orgs), always use a pony harness with a reserve tank, and regulator. If you do not have one then rent, or buy one.
Divers that like to dive deep, open circuit (buddy, or Solo) have certain commonalities. They do not talk about their dives, and they definitely do not brag about them - beware of those who do.
If you are in a course to get a cert, keep your mouth shut, and listen even if you disagree with your instructor. Instructors can be good, bad, or new. Just because they are new does not make them a bad instructor.
For example, when I meet a PADI DM that does not know that EANx was invented by NOAA, and released in 1995, it makes me wonder. IMHO, DMs that are cross-certified are best, but there are always exceptions.
If you are using EANx (Nitrox) know your decos, and RDP tables. Dive computers cannot get narc’ed - divers can.
Have I ever been narc’ed? No, but I have done a Solo drift dive from shore where I had to swim 6km back.