Seneca County is in Northwest Ohio. Of the 88 counties in the Buckeye State, Seneca County is 22nd in terms of its size (552 square miles). Browse all waterfront property for sale in Seneca County on the Land Network to find the marina, lakefront and other recreational property that meets your needs, from waterside land perfect as a retirement investment to existing residential waterfront land and property for hunting, fishing and other outdoor-recreation pursuits! The most common type of land for sale by combined acreage in Seneca County is house.