Depending on your style of play if you are the Serbs, you can either put your flotilla upstream to engage the AHs north of Belgrade or you can allow the AH fleet to sail down the river and engage them with the help of the three field gun batteries in the city. The AH has the responsibility of clearing the river of Serbian boats, bombarding the city to silence the guns and transporting their Marines, their troop laden gunboats are practically venerable to Serb fire since they have armor of zero.