Flippers and Paddling Gloves.  "His inventiveness appeared early; as a youngster he devised a kind of swimming aid - oval palettes for the hands and feet..." Source: A Biography of Benjamin Franklin

Paddling Gloves. "Ben Franklin loved the water. Growing up in Boston, he was drawn to the sea and often dreamed of becoming a sailor. Ben learned to swim and became an expert swimmer. Wanting to increase his speed in the water, Franklin devised fins that he wore on his hands. The fins were shaped like lily pads or an artist's paint pallet and helped the swimmer attain greater speed with each stroke." Source: Benjamin Franklin (PBS)

On a related note:
Gulf Stream. "Franklin was one of the first people to chart the Gulf Stream. Since he spent so much time sailing back and forth from America to Europe as a diplomat, he was able to measure different temperatures in the Atlantic Ocean and chart the Stream in detail." Source:  ThinkQuest Internet Challenge of Entries:  Benjamin Franklin, the American Renaissance Man

More info on Benjamin Franklin is available on-line at:
Benjamin Franklin: A Documentary History -- J.A. Leo Lemay