Generally they will be in water no deeper than 25-35 feet. They go to these areas for two reasons, slightly warmer water and if it's a Mud flat they will be eating worms or aquatic insects from the mud. They are generally only in these areas in the middle of winter and as others have said they start to migrate shallower as the water warms.
At night we catch them in 13 to 16 ft of water in or near weeds, especially cabbage.