Geocomposite articles that can provide a barrier against high conductivity water e.g., ocean water, are described and their method of manufacture, for waterproofing surfaces that contact high conductivity water. The geocomposite article mat includes a woven or non-woven geotextile sheet or mat containing a powdered or granular partially cross-linked acrylamide/acrylate/acrylic acid copolymer across its entire major surface(s). The powdered or granular copolymer has an unexpectedly high free-swell when hydrated with High Conductivity water, such as ocean water. A liquid-impermeable cover sheet is adhered to the upper major surfaces of the filled copolymer-carrying geotextile to provide a primary high conductivity water barrier layer that, if ruptured, is sealed by the swell of an underlying layer of water-insoluble, partially cross-linked acrylamide/acrylic acid copolymer.