I don't know about copper foil but aluminium (baking) foil is very flimsy and likely to be easily pierced. I would have thought the foil would have to be of a reasonable thickness to work and, at the price copper is it would be extremely expensive.

Old cheap nylon foam back carpet would probably work better.

I successfully lined a pond against horsetail and mouse/mole damage using cheap roofing felt . Done on a warm day it is surprisingly easy to fit to shape and is only about £10 for a roll 1 metre wide and 8 metres long at DIY sheds. One roll was enough to do my 3metre by 2.5 metre semi circular pond

Here's a photo

http://www.flickr.com/photos/31559373@N00/2683338208/