flip your boat on purpose then unflip it. climb up the bow or stern it will be easier. with a paddle boat or light oar rig you can flip it over yourself on the river. with a gear boat try to get it to the side or get a couple guys up there (with flip waist lines - the flip lines that tie you your boat invariably come out when you don't want them to and are a hazard). if the river is super continuous you may have to save yourself first and get to shore without your boat. if there are pools you will have an easier time.