Yep, every state is different. There was a big court case back in the 50's (Elder v. Delcour) here in Missouri that declared the high water mark of a "navigable" river is public property as long as there is a public access to it somewhere along it's watershed. Now land owners may interpret that differently such as the one the OP ran into, but in Missouri as long as you are between the banks, you can walk, camp, float, fish, and do whatever you want to do. Just make sure you don't get shot by a P-O'ed land owner who thinks differently.