Question

A vehicle that was a boat with tank tracks used to cross a coral reef

Answer 1

This was the Landing Vehicle Tracked (LVT), also sometimes called an "amphtrack" or "amtrac" -- short for "amphibious tractor."

The LVT was used in World War II, notably in the Battle of Tarawa.  Tarawa is an atoll, or ring-shaped coral reef, successfully getting soldiers transported across the reef and through shallow waters.