Governor Franklin D. Roosevelt of New York, Democratic nominee for the presidency of the United States, sailed into the waters of Dukes County late Wednesday, although not into the waters of the Vineyard. The Roosevelt yawl, Myth II, slipped into Cuttyhunk after logging fifty miles from Stonington, Conn., where the previous night was spent, and Mr. Roosevelt stepped ashore, his arms aching from a long day at the wheel.