Don
I will second Chris on the "plaster first" technique. On an area that large, pour the whole thing with a very thin plaster coat. You want to use paint to get the "depth" of the water effect you are looking for, not the thickness of the model water material.
This will also help seal the area because all of these water gels will find every pin hole and pour through sending this expensive stuff to the floor of your layout room.
I did a couple of small streams on my layout and learned the hard way the above. I planned it out where I would pour it at a high point and let it run down and look natural! Nope. I ended up with several really deep ponds and no streams..... This sucked up more bottles of this expensive stuff filling in all the gaps and depth than I want to mention on this forum.
Next time I will use more plaster and paint and use just enough of that stuff to make it look wet.
I think texture will be the main thing for you to focus on in some small practice areas before you start this area. In my small stream I wanted a more flat look. These resins will dry perfectly flat. In a big river bay like what you are modeling, you will probably want a mild wavy pattern. Scenic Express has a water effect product that is basically a clear caulk that is added on top of the resin water. It will hold a ripple. I did not do too good with it myself.
Steve DeLoach