For shallow streams and such where the bottom is visible, Envirotex is my choice...
Whatever product you chose, you're going to have to have the stream bed ready for the pour...
Bullet splashes are fairly easy to do... My technique is to wait until I pour the last 1/8th inch of water, then start working the resin with the tip of a pen cap (the rounded points of those cheap Bic pens are pretty good tools), until you have a three or four concentric ripples. Then grab some clear sprue and start stretching it. Cut and insert it into a hole drilled in the center of the rings and then cut it again about a half-inch off the surface. Using a lighter, match, or soldering iron, slowly move the heat towards the tip of the sprue and let it "ball" up the end of the piece...
Do the same thing with the pen cap around the rocks, but instead of circles, make the ripples elongated and close together on the upstream side with a little more space and pull down stream... Add "white-water" with titanium white judiciously around the rock... (Don't try it from memory. Have some photos to look at while you do it... ) Gotta work fast though, the resin sets up fairly quickly...