THE BRONX - The medical examiner is trying to determine how a man drowned off Orchard Beach yesterday. Police say the 50-year-old victim was discovered after a swimmer felt a body floating underneath him. Sources say the man may have suffered a heart attack and was lying on his back below the surface. He was rushed to Jacobi Hospital in cardiac arrest and was later pronounced dead. Lifeguards say swimmers should know their limitations, obey colored flags and wear life preservers when necessary. Orchard Beach swimmer rescued by lifeguards