I started out with a tiny pond that I am now using as my filter/waterfall. I then got a free intex pool liner, but the pond was only 17in deep....so I bought a 40 mill firestone liner and dug the deepest part to 3 ft so I can have some koi. I still need more rocks to go around the edges, but I am happy with how it turned out
It would have been easier to build the deeper pond in the first place....but I usually do things the hard way 
New pond:
old pool liner pond:
It would have been easier to build the deeper pond in the first place....but I usually do things the hard way 
New pond:
old pool liner pond: