In my game as CSA the yanks have squeezed past the two forts at Mobile bay and landed a sizable detatchment in Alabama. I figure they have to get supplied by sea. I have stationed two Ironclads in the mouth of the harbor. Shouldn't this stop all supplies from passing to these troops?
Yes it should, unless the region they landed in is adjacent to the sea via another section of its coastline that is not going through the river mouth where your Ironclads are keeping watch...