A moving blade in still water accelerates no more mass than is decelerated by a fixed blade in moving water. It should be evident that a fixed blade in moving water does not over time gradually aggregate more and more mass into its sphere of action or move it to some other place.
Boats DO NOT move forward because some mass of water is necessarily moved backward. Boats move forward because- purely as a consequence- the mass of the earth is moved backward; and whether the oarblade is connected to the earth through the inefficient agency of yielding water or of unyielding post makes a difference only to the efficiency of its action. In this process of moving the earth an ideal shell would move NO water backward. The perfectly efficient way of thus moving the earth is through the agency of a frictionless roller in a prepared and curving slot anchored to the earth, i.e., the tractrix defining the zero-slip blade locus.
When a blade disturbs ("moves") water the resulting work done is totally lost in turbulence and heat. A blade of 100 percent efficiency disturbs no water whatever.