There is game management on land, but overfishing and water pollution go on without sufficient control. It is our responsibility as divers to point out how beautiful and how fragile the world beneath the waves is. This is one major reason for this page. But it is also our responsibility to cause as little impact on marine ecosystems as possible. Avoiding to touch the ground by maintaining neutral buoyancy will prevent most of the damage.
Direct damage means breaking corals and kicking up sediments.
Sediments will settle on corals and plants and will reduce the amount of sunlight and fresh water they get. Just being there is an impact. Chose the place for your vacation by ecological criteria. Some organisms are harmful when touched, and your touch is no benefit to marine creatures. Fish are wild animals.