Someone just posted this link which could be your solution...

http://www.justliners.com/foamseal.htm

"Bernard Huckstep" wrote in message
...
Hi

I have a small pond (7' x 5' x1') with deep cracks in it. Pond water
gradually drains through the cracks. Does anybody know how to repair these
cracks and make the pond waterproof again?

Thanks in advance

Bonod