Dad used a similar method on his WW1 dioramas. He just layered varnish and paint, he would have been so impressed with your water.No problem.
Murky coloured enamel paint, greens or browns for the lake/ river bed.
As many layers of pva as you want applied fairly thickly with a brush avoiding creating bubbles. I think I used four or five. Wickes weatherproof pva as I know it dries clear & it'll also dry with a rippled surface.
Apply it neat, don't be tempted to water it down, it just makes it more difficult & leave a couple of days between coats.
Use polyurethane varnish to give a shine. Apply three thick coats. To avoid brush marks on the finaI coat, I dolloped it on.
Don't use water based varnish, it'll strip the pva.
When all dry, coat the bank with neat pva & scatter Woodland Scenics fine turf earth & then carefully apply 50/50 mix pva & water on top of the scatter, try not to let it run onto the varnish.
What I like about this method is that it's cheap, dries rock hard, it's hard wearing & can be easily cleaned. But you could probably leave out the pva (although it adds depth) & just use the varnish. Most of the expensive water effects will soften in hot weather, will attract dust & are quite expensive.