i use a 2 part clear resign, it is not air dry, so it will be hard as a rock in 6-8 hours, it can be colored just as easily as water effects, and can be poured to any thickness.
if you want to do rough water, and waterfall effects, simply wait 3 hours for it to get slightly hard, and then add it to the slope. it takes some pratice to do rough water, and falls, but i have had great su ccess with it so far.