A terrarium is designed to hold a shallow area of water. It is NOT designed to be completely filled with water. Depending on the species of frog you have, you can make the water shallow, around 1 inch(pyxies, horned, darts) or deep, around 5-6 inches(leopards, bullfrogs, etc). I wouldn't put more than 6 inches of water in it though. Terrariums use less silicone than aquariums and are therefore not designed to hold large amounts of water.
If there's any cracks in the tank, you can just use silicone to fix them. I had to repair a few of my aquariums last year for leaks, just put some silicone on them and they're fine