Think you misunderstoord the question Darefsky

from what I understand he bought a pre-made (i assume resin) river.
Easiest way I can think of is basecoating a very dark blue, then increasingly light drybrushes... depending on the sort of water you want you could give some parts a light green wash or something like that...
You could look up some dreadfleet painting tutorials on youtube as those models had water bases
hope that helps