For Americans of a certain age, the Mekong Delta is not a place readily associated with tranquil vacations. Nearly 16,000 square miles of wetland, it is a semi-wilderness of swamps, canals, islands and paddy fields that extend southwest for 200 miles from the outskirts of Ho Chi Minh City (Saigon) to the Cambodian border. During the Vietnam War it was a place of incessant, bloody conflict between Viet Cong guerrillas and men aboard the speedboats and helicopter gunships of the U.S. Navy and Marines. But today the delta is peaceful once more, a shimmering landscape decked out in a thousand shades of green, where farmers can harvest three rice crops a year from the rich alluvial soil.