You can take the intracoastal waterway in any size motor boat comfortably in less than 2 weeks. (One week if you are in a rush.) Any size will work, from 18 feet up to 50+ feet.
To get some exercise you could paddle a canoe or kayak in about 4-5 weeks. In 2010, Jack Stachovak made a 5000+ mile solo kayak journey around the Eastern United States. An adventure linking the Mississippi River to the Great Lakes via the Gulf Coast, East Coast, and New York Canal Systems starting and ending in the small town of Portage Wisconsin.
The part from Key Largo to New York City was 1350 MILES and took him from April to mid May. He followed a mixed route through the Intracoastal and open coast.
http://www.portagetoportage.com/