Two suggestions I can offer are clear silicon caulk, available at any hardware store, and water-based acrylic paint, available in tubes, at art supply stores or at more reasonably priced stores like HobbyLobby, Michael's or AC Moore.
The silicon caulk can be laid down on a flat wooden base that has been painted with your basic color, and can be painted itself, when it sets up. You can sculpt it to make bow waves and wakes, and swells.
The acrylic paint can be used to produce a relatively calm water surface. One of the guys in my club (Delaware Valley Scale Modelers), Bob Ciccone, gave me that tip, and he uses it for some of his 1/700 builds. Apply the paint right from the tube, without thinning, and it's thick enough to allow for some sculpting with the brush. You won't get really deep swells, but like I said at the top of this paragraph, more of a calm surface.
Either application can be used over a plaster or Celluclay base, too.
Hope that helps, best regards,
Brad