Don,

Water is much MORE easily compressible than rock. That is why P waves travel slower in water than rock. The velocity decreased with increasing density, and increases with increasing stiffness (incompressibility).

Also the shadow zone is just a shadow for the direct waves P and S. The surface waves L and R, and many other phases like PP and SS, have no or different shadow zones.

But most of your post is accurate. Happy Thanksgiving.

John