Troop-carrying amphibian tanks being built for the Untited States Navy were recently put through their paces in the Gulf of Mexico by Marines, who reported the craft easy to handle in mud or water and capable of going anywhere land tanks can travel. The sea-going tanks, invented by Donald Roebling of Clearwater, Fla., can carry 40 men at nine miles an hour in water and 25 miles on land. Originally developed as a means of getting assistance to communities marooned by hurricanes, the tank was equipped with track-laying tread which had large cleats serving as paddles in water or mud. The Navy's machines are 20 feet long, 8 feet wide and 6 feet high.