Passengers on one of Iowa’s Mississippi River gambling boats were evacuated yesterday after a barge the boat was moored to sank. Ken Bonnet, a spokesman for the Clinton-based Mississippi Belle II, says 176 passengers were escorted off the boat over a rigged up gangway. Bonnet says the barges that support the gambling boat were checked at two o’clock Monday and had no water in them.By four o’clock, the barge was listing. Divers began working on the boat’s hull to weld up the hatches and places that were damaged when the barge pulled away. He says they aren’t sure what happened.