Robert de Gast, 79, a photographer whose 1970 book The Oystermen of the Chesapeake captured in harsh and unsentimental images the final days of America's last fishing fleet under sail and is regarded as one of the finest depictions of the watermen who make their living there, died of cancer Sunday at a Baltimore hospice. Dutch by birth, Mr. de Gast spent most of his life as a freelance photojournalist and commercial photographer on the shores of the Chesapeake Bay.