You can drag a box from the catalog and resize it. Just set it's height to 1 cm or so, doesn't really matter since the actual height will disappear anyway.
When you modify the box you can click the button "Materials" on the bottom left of the dialog. It then shows 6 sides for materials: top, bottom, left, right, front, and back. You can make them invisible in the same dialog suing the radio button "invisible". Only the top should remain: click the button "Texture" and from the texture list select a water texture (type 'water' in the little box at the bottom of the list). Click OK, OK and you can export the box.
Set the d value in the MTL file that results from the export. Import the OBJ file and you have your water.