Always dive in pairs. The time-honored "buddy" system saves lives.
Don't use ear plugs; they tend to cause ear ruptures from the inside out. You can learn to clear your ears naturally.
Always use a float. Inner tubes or paddleboards can be used to rest gear and to stretch out stricken divers for all-important respiration exercises.
Avoid homemade breathing tanks; some have been shown to contain carbon monoxide.
Handle spear guns with the same care you would other deadly weapons.
Use quick-release buckles on all weight belts. Be ready to jettison equipment; nothing should hinder a fast ascent when necessary.
Plan some effective method of two-way communication between divers and watchers.
Look up before surfacing. Jagged rock ledges and masses of kelp can loom up unexpectedly.
Let the air out of your lungs steadily as you ascend. If you hold your breath, the air expands in your lungs.
Dress simply; you're not auditioning for a frogman movie. All the novice really needs is trunks, a float, flippers and a mask.