Press barons, sailing, 21st-century life, and more; Aboard a floating fish factory; Distant Water, by William W. Warner. Boston: Little, Brown & Co. 338 pp. $17.95.

The world's ''distant water'' fishermen are going the way of the species they pursue: Their numbers are dwindling. Since the 1950s, when the advent of the ''factory-equipped freezer stern trawler'' freed them from the need to return fresh fish to port, these fishermen have combed distant seas with awesome efficiency. Once, they were hailed as heroes in the war against hunger. But today they are denounced as ruthless exploiters of the environment and are hemmed in by 200-mile national fishing zones.

After writing a Pulitzer Prize-winning book about the sea creatures and waterman of the Chesapeake Bay, William W. Warner became intrigued by ''the other end of the scale, the ultimate fishing machines.'' So he voyaged at length aboard the factory trawlers of five nations - Britain, West Germany, Spain, the Soviet Union, and the United States. With an unblinking eye for detail and unerring ear for language, he captured the flavor of the fishermen's way of life - their daily work, their squabbles, celebrations, memories, and dreams.