Several years ago I built a traveling display for Sea Port Model Works. (There are photos of the display base and several structures shown in earlier postings.) The question that everyone who sees the display asks is "how was the water built?" To answer the question I put together a 10 minute video showing the process. You can view the video here.
At the end of the presentation I talk about sealing the water surface using Gloss Acrylic Varnish. This is something you need to get in a paint store. Here's a photo of the brand I use but any good quality water-based varnish will work.