Styron said. You are on private property, but you have a right to be there if its a navigable stream and as long as you are on a gravel bar that is submerged during parts of the year, because its part of the stream bed.
If you don’t want floaters, don’t buy riverfront property on the Meremac river.
If he really wanted to stop people from urinating along that gravel bar, cameras and photos of perpetrators loaded on a web site would probably do the trick.
I am sorry, but it takes a lot more then being able to float down a body of water on an innertube to make it navigable.
It also funny I never realized that property owners rights depend on where the property is located or the level of desire of the public to use it for their own purposes.