My favorite part of creating model pools and water features is adding details that suggest running water. These are much easier to create with artist's acrylic mediums than with any of the commercial modelling materials. My favorite material for these features is Tar Gel Medium.
Tar gel medium forms thick, ropy strings, when dripped off the handle of a paint brush onto a model surface. To make my small waterfall, I dripped stings of tar gel from the top of the rock edge above my pool, using a toothpick to draw the strings down to touch the pool surface. The waterfall was built in three applications. I began with two lines of tar gel which were allowed to dry, connecting the top of the rock to the surface of the pool. Then I added more tar gel to get the 'arch' of the water out from the rock to the pool. Finally I added tar gel to the top of the rock to give me flowing water leading towards the pool. As this is a 'garden' pool, the water source is hidden behind a group of rocks and plants as it is in most gardens.
When the tar gel had completely dried I added a bit of frosted glass stain to the base of the waterfall to mimic the action of foaming water. Unlike silicone, which dries with a tacky surface and often attracts dust, tar gel dries clear and firm, but remains flexible. It is great for wet footprints and spilled water effects as well as it can be teased back up into 'frozen moments' splash lines.