I prefer the sculpted/fixed boards myself. If you make it at least semi-module you can mix the board up quite a bit and then use individual terrain to make each battle unique.
I've had the best luck with Testors Gloss for water effects. Just pour the stuff straight from the bottle and use a brust to spread it out. It goes reasonably far (it's cheaper than buying the wax water effect stuff) and I'd guess you could probably do that whole river with a single bottle of it if you stretched it, or a bottle and a half if you were a bit generous on your layers. My buddy used a spray effect gloss that kind of worked, but really didn't turn out quite as nicely.
Kind of hard to really do the water parts justice with my photography skills, but this will give you an idea how it turned out.