River current changes considerably on whitewater streams and in large open bodies of water, especially on the ocean.  When the river is wide, the current is pretty slow.  If you flip and swim it is easy to swim and tug your boat to shore.  When the river necks down, the current picks up significant speed.  Water can  start piling up on river banks that you probably want to stay away from.   Picture a weight at the end of a rope.  As you swing this rope around you, the weight has to move a much larger distance than your hand.  The same is true on a river.  Current is much faster on the outside of river bends.

Ocean currents can be quite deceptive.  Some areas can develop rip tides which can be rather forceful and dangerous to swimmers.  Here is a good set of articles on this phenomina: Rip Tides.

River current doesn't always flow downstream.  When water drops over ledges and around large boulders, eddies are formed.  Water current at the surface works its way back upstream.  Between the eddy and the main current is an eddy line which is well defined at the top of an eddy and far less stable further down the eddy line.

In chutes, water current is typically fastest in the center.  Once the tongue runs out, energy is disapated in waves.  Down river racers run on the shoulders of these waves to increase their speed.  If they run too far on the side, their boats will slow down considerably since they will have entered slack water formed by eddies.

Currents can be quite powerful in moving water as the following article explains: Current Power.