First, never dive alone. Always have a diving buddy in the water with you.
Second, never dive without checking and double-checking your equipment. Keep a pre-dive checklist to ensure that you have everything you need and that it is all in working order. You never want to dive with incomplete or damaged equipment.
Finally, don’t disturb the wildlife or the habitat. This is an important part of dive etiquette. You want to maintain the beauty and richness of the environment for other divers to enjoy.