Great Stuff is this insulation type of stuff. You can either use something like cardboard to shape it and spray the GS over it, or you can mold it while it's wet, or once its cured you can carve with a serated knife. For water features people will either add rocks/pebbles while the GS is still somewhat wet (enough for them to stick) and then silicone it, or they will cure the GS and stick rocks/pebbles in the silicone for a natural look. I've seen people put coco fiber and other materials into the GS or silicone as well. GS by itself won't work, but it's pretty hardy stuff and is fairly easy to make what you want out of it. Here's a picture of somebody's water feature they made with it.