Good quiz. For Kentucky and West Virginia, it's not the Ohio River but the Big Sandy River, and you could also probably accept Tug Fork because that fork of the Big Sandy forms part of the border too.
There are two rivers that separate Florida and Georgia. In addition to the St. Mary's River, the Chattahoochee forms the border for about 35 km just NW of Tallahassee. Once it crosses completely into Florida, it changes its name to the Apalachicola River, but I suppose that name could be used, too.