RedactedNarrative: Diver was diving without the proper marking flag within the channel.  Vessel operator stated he was traveling west to east under the bridge at a slow rate of speed.  He felt the vessel strike an unknown object and placed the vessel in neutral.  Operator then heard a air hissing noise and saw a diver appear.  The diver was helped aboard another vessel in the area having suffered a laceration to the back of his head.