Quick story. My partner and I pull up to an island. five feet away he jumps out of the boat and can't touch bottom. we figured out there was a steep underwater ledge just off shore. Three to ten feet deep in one step.
One of my spots is like that too, only it's about 10-15 feet off the island. Tell you what though, when it's dark and/or murky it's easy to misjudge the distance.