RedactedNarrative: Vessel 1 was occupied by three people and pulling two people on tubes traveling south at about 20-25 miles per hours when a "popping" sound was heard and the engine shroud moved forward about 12 inches.  Flames were immediately visible.  A fire extinguisher was deployed by the Operator which had little effect on the fire.  The three boat occupants jumped into the water.  All five people involved were picked by other boaters and transported to shore.  The boat burnt, broke in half, and sunk.  There were no injuries.