If your ship is standing at a shallow/crossing where armies can pass over then the enemy shall be hindered from using it.
This is nothing that I've ever ended up testing for real (aka I never seen an enemy army not been able to pass, though perhaps they havent gone near because they couldnt go there with my ship in the way?) but that has been constantly reported and I thus acknowlidge it as a truth.
Edit: and welcome to the forum