Under water earth quake is an earthquake that occurs underwater (at the floor of the body of water). Underwater earth quakes affect the ocean floor greatly. Underwater earth quake pushes up powerful waves and unleashes underwater landslides which cause tsunamis which in turn destroys corals reefs and damage sea life of the ocean floor.
Keep in mind that corals like to stay in quite waters with little or no sediment because corals require sunlight to survive. That is why the turbulence from underwater earth quake affects coral reefs greatly.
The magnitude of such earth quakes is usually very high. The earth quake is mostly very destructive within the vicinity of the quake.