@blattacker Ever since I posted that comment, I’ve been working on adding a lake to one of the islands I posted earlier in WIP section. Using the method described in @hylk’s tutorial.
Turns out it isn’t that straightforward when working with multiple water sources, at different height levels, on an existing terrain. I was able to figure it out eventually (two full days of hair pulling math…)
Made a thingy, out of another thingy… 
Made a water combiner macro that at least works for this project, and accounts for different height levels. Don’t know if I’d be able to make it work for every use case, yet.