I think EVERY diver should know THIS:

[aquaregia]

A good diver knows when to call a dive.

 

[pieohpah]

Too many things... but be safe and keep others safe.

If you're swimming in shark infested waters, be sure to be a better swimmer than your buddy.

 

[Tanked]

The diver with the most C cards isn't always the best diver.

 

[stairman]

Take a cavern course,and pass it. Even if you never dive a cavern the skills learned will make a difference.

 

[=C=]

Assuming we were going for just one answer each:

1. One rule every diver should know - Breathe regularly. Don't hold your breath.

2. Every diver should be able to - Avoid an OOA (monitor air supply)

 

[gypsyjim]

Diving is fun. Pay attention to the little things, but do not over complicate it, and enjoy the experience.

 

[Walter]

Avoid an OOS

OOA = Out Of Air
OOG = Out Of Gas
OOS = Out Of ????

A diver should have mastered skills to the point they are automatic. No mask breathing is as easy as breathing with a mask. Every diver should be able to easily handle the failure of any two pieces of equipment at the same time.

Every diver should know that it's much better being on shore wishing you were diving than to be diving wishing you were on shore.

 

[ScubaSteve]

OOA = Out Of Air
OOG = Out Of Gas
OOS = Out Of ????

My guess is he intended OOA as the A is right next to the S or he meant Out Of Stupidity (hence a smarter diver )

 

[h90]

Every dive with an instant buddy is a solo dive
If you have a problem no one will help you.

 

[scubadiver888]

Every diver should know that it's much better being on shore wishing you were diving than to be diving wishing you were on shore.

Is this your phrase or are you quoting someone else? Regardless, I like it. Do you mind if I use it someday?