Interesting. Good analogy, and it IS easier to visualize with an aircraft. So, with momentum, the force of the air/water pushing the front of the prop aids in increasing the RPM and reduces the load on the motor. Correct? Is this what you mean by angle of attack (as the pitch of the prop isn't changing, (with the boat anyway))? Makes sense, but I never thought about it (obviously, physics isn't my strong suit!).
Think of it more in the way of sailing you have actual wind, what you get if you feel the breeze standing on the shore and the apparent wind you get when you are sailing, which is a vector of the wind from it's true direction and from what you create by your own forward movement.
So, if you prop is turning (under engine) but your boat is not moving through the water then it's leading edge of the blades is getting maximum resistance from the water, trying to "cut" the water if you like.
As your boat makes forward motion the prop is "cutting" the water at a different angle (remember the wind vector analogy) and this reduces the load on the engine.
I've never had it explained to me in the terms of the air/water "pushing" the front of the prop, however that could essentially be correct, as when we stopped an aircraft engine in flight and then reduced our air speed, at some point the prop would stop "windmilling" and be stopped by the compression of the motor.. If we then dived to high speed we could usually (not always) forced the prop to begin rotating again, so I guess you could say it was pushing the prop.