Stephenson:Neal:Quicksilver:91:Atlantic is striped with currents (Chris Swingley)

From the Quicksilver Metaweb.

The existence of warm and cold bands of ocean water was first recorded in 1612 by Lescarbot: "I have found something remarkable upon which a natural philosopher should meditate." The use of the thermometer as an instrument of navigation is credited to Ben Franklin in 1775 and Charles Blagden in 1782. Franklin carried out rigorous ocean temperature experiments on many of his ocean voyages and produced a reasonably accurate chart of the Gulf Stream with his cousin and Nantucket whaling capitan Timothy Folger (1770).

At the point when Waterhouse is making these measurements, several studies of ocean currents had been published, and measuring ocean temperature would have been part of a very preliminary investigation.

For more information, consult:

Ben Franklin and the Gulf Stream

Benjamin Franklin and the Gulf Stream

Henry Stommel. 1950. "The Gulf Stream: A Brief History of the Ideas Concerning Its Cause". The Scientific Monthly LXX(4): 242-253.