BAGHDAD, Iraq - Four US Marines drowned when their tank rolled off a bridge into a canal, the military said on Friday. Gunmen killed a Sunni preacher and his son in the mostly Shia city of Basra despite an appeal for tolerance by Iraq’s top Shia cleric.The four Marines died on Thursday near Karmah, 80 kilometers (50 miles) west of Baghdad in Anbar province, a focus of the Sunni Arab-dominated insurgency. A US statement said the accident was not due to hostile fire. “We are a close-knit family and this loss affects us all,” said US Col. Larry D. Nicholson, commander of Regimental Combat Team 5. “Our thoughts and prayers are with the families of these Marines during this difficult period.”