There are various materials one can use to make "fake water" and different ways to do it. For example, one could paint the bottom of a depressed area with acrylic paints to look like the colors of the ocean or whatever kind of body of water is desired, then pour a layer of epoxy resin in it (your friends would probably want the "hard"-surface type of epoxy resin), or various kinds of other clear meltable crystals, etc.... some of those can even be disturbed in various ways while curing to create the appearance of "white" water, or waves, etc.
Resins can be tinted any color with artists oil paints or with purchased resin colorants (the oil paint colors will generally be transparent unless white is added which makes any color opaque, or the purchased ones come in transparent or opaque versions).
You don't say how "professional" it needs to be though, so crumpled blue and green cellophane over a brown-painted depression could work too, etc. for quick-and-easy. Or they could use a large sheet of acrylic from a hardware or plastics store, etc.
Otherwise, check out this page at my site for lots more info and also links to other places that could give more info for large dioramas:
http://glassattic.com/polymer/other_materials.htm
....click especially on the subcategory called MAKING FAUX WATER which is about halfway down the list at the top of the page
HTH,
Diane B.