Energy conversion efficiency - desired energy output per energy input

If you go here:

http://en.wikipedia.org/wiki/Efficiency

You see how many versions of efficiency there is.

When you go up a hill your car goes into low gear because it strains the motor left in the higher gear.
It becomes less efficient.

In paddling, the human body has to be taken in to consideration when one is to assume that efficiency is the least motion to move something. If I was challenged to move my boat the furthest forward that I could with one stroke, I would pick the most efficient paddle I could - probably one with a blade twice the size of any made.

But if I had to use that paddle for an hour, I would probably die pretty quick. It would not be very efficient for me if my goal was to paddle for several hours. Bicycle racers always pick the lower gears to keep from taxing their muscles from strain. Any bicyclist who uses too high of a gear will tire out quickly.

So Nick's right.
"As a general principle it is more efficient to generate a propulsive force by pushing a large mass slowly than it is to push a small mass quickly".

The most efficient paddle is one that you can generate a strong propulsive force for the period of time that you would like to do that.