I'm building a large pond and am in the process of rocking it in. I'm using both cement and "waterfall foam" for everything underwater as it is "fish safe." It comes in a similar can but for a much higher price. It, too, is black, which does help make it unobtrusive. I wouldn't use this for in-the-pond cementing as I don't know if it is biologically safe, or made to last while permanently underwater. However, I also have a lot of rock work on walls that don't get wet (unless briefly when it rains), and here I need something really sticky to glue the stones in place, and fill the voids behind them. It takes some practice to get the right amount into a crack -- not too much or it will expand and spew out, all over the place. It's better to be conservative at first, and go back and add a little more if necessary, until you get the hang of it.