I have a neater, cleaner, and more portable solution to water jug testing. Pack old newspapers into a plain old 5 gallon plastic bucket. Add a couple drops of dish soap and fill with water. When all the water soaks in fill again until its all saturated. Most defensive-type JHP ammo will stop in the bucket, if you're firing something like a hunting round or an FMJ you can stack two buckets. It will last for dozens of shots, but eventually it will get shredded enough that it's hard to find the mushroomed bullet. One full bucket can test more rounds than 20 or so full water jugs, and the logistics are easier. When you're done the newsprint can be dried out and recycled.
The last thing I saw fired into a pool was a 12ga flare, and it stopped in the water with no damage, but that was a beer inspired college experiment.