Do not touch wildlife. Avoiding wildlife is safer for you and better for the wildlife. Some marine animals have a protective coating that is rubbed off when touched, exposing them to parasites and infection. Touching or "playing" with them also stresses the animal.
Coral are marine animals. Take care when snorkeling or scuba diving and avoid touching, grabbing, or accidentally grazing the coral with your fins. The portion of the coral that is touched will die.
Do not take marine animals out of the ocean for any reason. This also stresses the animal.
When taking pictures underwater, do not touch the animals. Do not lean or hold onto coral or other underwater structures while trying to "get a good picture." Remember, touching coral in any way damages it, and the portion damaged will die.
Secure "dangling gear" that may damage the reef. Secure the secondary air source, computer console, flashlights, or any other gear that may come into contact with coral and other marine life.
If you witness someone abusing a marine animal, contact the authorities if the information is available. There are many reefs that are under marine law protection (for example, the island of Bonaire) and have organizations to protect the animals.
You are in their environment and world. Respect marine life as a living being and take only pictures, and leave only bubbles.