If you are really keen on this, some researchers at Memorial University of Newfoundland (Canada)have done a lot of studying of these "Oscillating Foil Propellers" (plug that into your search engine for more info), including measurements and theoretical studies on whales and performance estimating programs. Its a difficult field aerodynamically, because the blades have a constantly changing angle of attack during the stroke, while all the airfoil data are for steady state conditions.

To generalize, rigid blades can provide more power but flexible blades are more efficient.

A couple of ex-graduate students have/had a web site devoted to this, but I don't have the address.

I have some references; e-mail me and I will pass them along when I get back.

david