I'm not sure about the plywood (although I don't see why it wouldn't work), but you can make any pond you want out of brick or cement. Then, there's a company called Kelly Technical, which sells pool epoxy that you can paint it with to make it totally water proof. First, one coat 'gunzite primer', then one coat 'zeron epoxy'. The cement has to cure for 30 days prior to painting though. A gallon of primer and epoxy does approx 150 sq feet each, and cost $75 each. You can get the epoxy in a nice blue color. You can find their website on the net. Hope this helps.
Mike