Fast-flowing river estuaries virtually do not exist, and the points of contact with ocean and land are essentially flat surfaces.
Also, there is usually a long transition between freshwater and seawater, and freshwater fish do not swim forward when they get there.
Even if the situation you mentioned occurs, there is no need to worry about the fish. They are not as mentally retarded as you think when they are thrown back into the sea.
Fish swimming in water is like us walking on land. When we reach a steep slope, it is usually impossible for us to roll down, and the same goes for fish.