Hi Cookinghead, thanks for your comment. I’ve only just approved it because I’ve only just seen it – having been away from the internet for a few days (these days that means not even a mobile signal!)
The pond is about 18″ deep, I think. It’s actually a preformed pond with a special “wildlife” edge along one side, formed by sort of bog container along the edge. I believe the frogs appreciate that this is a pretty cool way to get in and out of the pond, as well as looking better than artificial edging. Liner detail here.
Unless you want a deep pond in a small area, line the pond with flexible liner. If you want a pond, rather than just a watercress bed, don’t go for the cheap stuff. Butyl was the in thing when I researched this about five years ago. Concrete is a lot of hassle, and from experience, I can tell you it’s quite hard to repair, though I do think it can look great.
However if you just want to grow the watercress, I have to report it does seem to grow anywhere, as long as you keep it pretty wet. Just do be careful where you grow it and be mindful of the health risks!