I can tell you, from running many whitewater rivers, creeks, and sea kayaking places with lots of current that there is no rule saying that the deepest, swiftest part will be on the outside of a turn. In a sailboat, most of the rivers you run will have been altered by dredging or boat traffic. Local commercial boat traffic is often the major determiner of where the "channel" might be. You really have to take a moment to look carefully at the surface. In observing moving water, a chain of regular waves, not always very obvious, will indicate where the deepest, swiftest part is. Just moving to the outside of a turn will very often bring you into shallow water because you are simply closer to shore. It may look faster because of turbulence caused by rocks. Rock-strewn, ledgy river beds are not readily shaped by current and you just can't use a rule of thumb.
Alberg 35: With a philosophical flourish Cato throws himself upon his sword; I quietly take to the ship.