Cement, especially fresh cement, might leech nasty stuff into your water. I put fresh concrete blocks into my turtle pond once, and two of the turtles died within days.
If you still want to do it, several magazines have recently had articles about fabricating concrete objects. One was DIY (Do It Yourself) magazine by Better Homes & Gardens, a recent issue. I think it was Martha Stewart's Living that had another aritcle. Google it. Or just Google the instructions. Also, a year or so ago, we had in the Chit Chat forum, instructions on making leaves from concrete... might work for a tree limb, too.
On a slightly different topic, I saw on Thalassa (a French TV show about the sea and seaside living) that in the Panama Canal, there are whole forests that were submerged when the Canal was built. Now people are cutting down the forests to 1.) clear the canal pathways, and 2.) harvest the wood. Amazingly enough, the wood that has been totally submerged for decades is still perfectly good and useable. Only the bark is rotted; the wood is fine. Expensive proposition, but the wood is valuable and sought after.
