Why do you want to use a cement based waterproofing product? There are several epoxy or liquid EPDM products on the market made specifically for sealing concrete ponds. These products are FISH-SAFE, so there is no guesswork. Epoxies include Koi Kote and Pond Shield or Herco H-55 if you prefer liquid EPDM.
Personally, I would demo the concrete and install a 45 mil EPDM liner. Less expense and maintenance in the long term. Concrete ponds WILL crack over time and no matter how well or expertly the are repaired will continue to crack.
"Everybody is ignorant, only on different subjects." - Will Rogers