It leads to a reduced ability to capture natural food, makes them dependent on people and they lose their natural wariness of people, leading to aggressive behaviour.
It may also be harmful to divers especially when attempting to feed fish with poor eyesight but a strong sense of smell! e.g moray eels, groupers, they might easily mistake your hand for the food!

Don't litter the ocean; even biodegradable items such as banana skins and orange peel can be mistaken by fish and marine mammals and shouldn't be thrown overboard. Worse still are plastic water bottles, snack wrappers, cigarette butts and waste food. Take a carrier bag with you and offer to collect rubbish for safe disposal.  How many times have you seen divers and boat crew throwing their cigarette buts into the ocean?

Take nothing out of the sea, except recent litter; use your judgment - most floating items pose a threat for marine life - for example plastic bags cause a significant hazard to turtles that confuse them for jellyfish, ingesting them and then dying. However some older debris (old tires, dead sea shells) may already have formed habitats for some marine life.

Practice good buoyancy control; peak buoyancy control is key to enjoying a relaxed underwater encounter minimising the risk of contact with corals or rock formations.

Adopt appropriate finning techniques for the conditions; mid water column diving might warrant a regular kicking motion whilst a frog-kick or gentle flutter kick could be more appropriate for closer proximity to corals and the sea bed to avoid kicking up sediment.

Ensure all equipment is well secured so that it doesn't drag or catch on corals; being able to locate your gauges and octopus without looking is not only sensible diving practice it avoids items knocking or snagging on corals.

Adhere to all local dive rules and regulations; each dive location may have separate rules that should be acknowledged. Find out and adhere to these guidelines for safe and enjoyable diving.

On land / boats

Encourage and support the use of dive moorings; choose and reward dive operators that use fixed moorings at dive sites. Dropping an anchor overboard can cause monumental damage to coral reefs and the surrounding environment.

Learn more about coral reefs, fish and marine creatures; the more you learn, the more fun you can have sharing your knowledge and identifying items underwater and, chances are, you'll understand more how precious these creatures are.

Don't buy souvenirs that feature items taken from the sea; corals, sea shells, starfish and sponges have been pilfered from the sea to make gaudy souvenirs and trinkets. These items belong in the sea, not in the bathroom.