i believe you are entitled to use ALL the lake. it's the "navigable waters" part of an act. it started when farmers/ranchers would close off a river, not allowing boats, rafts, or fishermen access to their "private" ranches. the courts said if a boat would float on it, or you could walk it, it was not a part of the farm/ranch but was community waters. you CANNOT step on dry land, above high waterline, but you can walk/float any water the boat will fit.
short point is, he is wrong and you have a 1/3 interest in that lake. check with the local law, of course. but you can actually open a travel lane on your property, and allow the PUBLIC to fish that lake. perhaps just a jab at him, but in no way does he own the waters and his property, by federal law, ends at the high water line. yours does, also