This is a thought I asked myself often, but never did real efforts
to get an answer. Barsmonsters question about number of fans of a
wind turbine made me think of it again
Why do blades of aircraft propellers or wind turbines cover only a small fraction of the area they circle? Propellers of ships or Kaplan turbine propellers cover almost the entire circle. (Independent of number of blades). Same for steam turbines and jet turbines.
I am shure that the forms of those propellers are very close to optimum, due to decades of
experience.
Best Answer
An engineering answer:
Note that it's not just about water vs air. It depends on a lot of things: the density & compressibility of the fluid, and tradeoffs between torque, efficiency, cost, materials, maintenance needs, fouling hazards, and so on. Below is a water turbine blade, the SeaGen, that's not much different from a wind turbine blade, because it's doing a similar job, with similar constraints, but in water rather than air. But bear in mind that this is (AFAIK) the first tidal turbine to achieve commercial grid operation, so later designs could diverge.
Some blades are indeed close to optimised given the materials available at the time, thanks to decades of experience. However, new materials can enable further optimisations to blade design.