Quote Originally Posted by brian 333 View Post
By using shallows, backwaters, and eddy currents one can easily pole a barge upstream, and portage around areas where such advantages do not exist. This was how the rivers and (later) canals became the highways of commerce in Europe and America two centuries before the advent of steam engines. (And long before in other cultures.)

Certainly currents in the ocean have eddys and backwaters. Indeed, if one were to look, one would see that not only do surface currents make gyres, or huge circles around the ocean, eventually returning to their point of origin, these patterns create smaller, regional gyres, and not only in the horizontal. Currents in the ocean are 3D. It may be possible to ride the Gulf Stream north and the Arctic Haline Current south or to use the Loop Current in the Gulf of Mexico to go from any coast to any other coast in the Gulf.
Physics are not my strong suit. I understand that an ocean current would create forces on the edge pushing the opposite direction, but I'm lost on how a Merman would pole a barge or the functional equivalent of a barge against a current. Please elaborate.