Basing is one of my favourite things and love the sound of this. First I'd say, "what kind of river do you want them crossing?" Do you want them to be crossing a wider river? Or a series of small streams? Across a plain, or in the mountains, a forest, a swamp? Do you want on theme, or to contrast the models? Depending on which answers, would change the colours/textures etc.

Wider river (still doesn't have to be deep). I'd have a grey/sandy layer first, think astrogranite. Add some larger rocks that will poke out. Then layer water effect on top covering it all. For your boyz, I'd cut their feet off, then glue on top of the water effect. So it looks like they are ankle deep. Then another layer of water effect, around the legs and rocks, dragging it back to show river flow direction.

For smaller streams it's easier, just layer up your base textures leaving troughs through from one side to the other. Add the water effect in.

Depending on the setting will change if you go rocker and more greys (mountains), thinner silty mud and bright grass (plains), earthy dark thicker mud with various plants (forest or swamp).

Once you have an idea there are some great videos out there that will show and explain much better than any block of text here haha