Except where a dividing line is fixed in a river by law, the jurisdiction of a county lying on a navigable river in this State extends from the shore to the channel of the river dividing the county from another county. Except as provided in subsection (c) of this section, if a vessel is located on a river between counties, process may be served on board the vessel by an officer of either county. A vessel that is moored or fastened to the land on either side of a river is considered to be in the county where the vessel is fastened.