I use Battlefield in a Box woods as trees for mini games, but I don't use the flat template pieces that the trees are supposed to sit on top of, so I thought, why not turn one into a lake? I flocked patches of it with static grass, then painted Mordian Blue blobs, and dabbed my brush heavy with water to spread it around. Then while still wet, I thickly did Ultramarines Blue around the shallow areas. Finally, I brushed some Desert Yellow where the grass meets the water for a sort of shoreline look. I haven't tried painting water terrain before, so I'm happy with how it turned out. I might get some gloss coat or something and give the blue reflective shininess once it is all dry.
With a Battletech mech on it, it's a lake.
And with a guardsman and sentinel, it's more a large pond in 40k scale, but a nice size :)