Interference can cause erratic depth readings.
If this happens again, turn off everything on the
boat except the sonar in question, and see if it clears up.
Also, I have to ask:
Is it possible you were anchored on a ledge, or on the outside of
a hole, where your anchor was in 5.5 feet of water, but the transducer was
looking down over the ledge or into the hole?