Sails/wings create lift (a force) by altering the momentum of the
air passing by.

yes.

for the rudder to create yaw, it must deflect water.

yes.

It
must change the momentum of the water.

yes.

That's where I'm stuck

yes.

I see the rudder (prop in
reverse, boat static) altering the direction of the water
approaching the prop.

no, the water pressure of either side of th rudder is the same.

Now, perhaps it doesn't

it doesn't.

perhaps there's an opposite effect
somewhere else which I haven't yet identified

the water pressure on either side of a rudder is the same for water drawn over
the rudder.