Abstract: A panel, and method of making the panel, useful as a water barrier including an intermediate layer of a water-swellable colloidal clay, such as bentonite, sandwiched between two layers of sheet material, such as paperboard, wherein at least one of the sheet material layers has a plurality of spaced apertures, or water channels, extending from the exterior of the panel to the intermediate water-swellable clay layer. The sheet material layer containing water channels includes a coating of a removablel material completely across the water channels in the facing sheet having controlled, predetermined water-solubility so that the intermediate water-swellable clay layer is quickly hydrated after solubilization and removal of the coating material, and to prevent clay hydration during installation, prior to removal of the coating material.