Ray,
I like your idea!
One problem with it that I see is.
Lets say the windmill is on the bow of the boat and the wind is blowing at it to make it turn. Now the shaft to the out drive is turning the prop. You would need a gear box to turn the prop at least twice as fast as the mill to only move the prop. Now with the wind in your face and the current I think you will go backwards. If you change the ratio, that would get the prop going faster, but if you don’t have a gear shifter, you won’t get the boat to start moving.
Maybe a solar panel electric motor to get it started moving. Then switch over to wind mill power.
Maybe instead of the wind mill, try a stick mounted to the bow of the boat and tie a sheet on it and let the wind move you that way.
If you do this you might want to remove the out drive for less weight. Just remember to plug the hole!
Good luck with your invention!