I planted my waterfall with watercress one year. You have
to watch it as it will cover the whole thing. But the roots
are shallow and brittle and it is easy to rip up.
I bought a bunch at the grocery store. Sometimes it is sold
with the roots attached. Or not. Even the rootless stuff will
grow roots quickly enough. Heck, you can even anchor down
the leafy end and it will grow roots. The stuff is tough.
Plant by laying down the sprig and putting a rock on top
of the stem.

For edges creeping jenny works well and will grow out into
the water. I've even had an island filled with creeping jenny.

k :-)