Avoid kicking up the sand. It spoils the visibility for other visitors and damages corals and other reef animals when it settles. Do not spearfish. This is now prohibited in most countries and spearguns are usually confiscated by Customs officials. Leave all corals and reef animals where they are.
Corals are the 'building bricks' of the reef.
In many areas shells and other reef animals have become rare because too many people are taking them. Many tourist resorts and dive operators ban collecting. Take great care in underwater caves and caverns. Avoid crowding into a cave and don't spend too long there. Bubbles collect in pockets on the roof of the cave and the delicate reef animals there can 'drown in air'.