As a dog owner, I have water buckets that hold about 20 gallons I would guess. The dogs don't just drink out of them, but down here in MS weather they get in them...splash around...and get out. They have held up for YEARS. What are they...
Cow "molasses buckets." They are much heavier duty than a 5 gallon bucket. Forget the kiddie pool idea...way too thin and are not built to last. The cow molasses buckets are very tough, thicker than a kiddie pool or 5 gallon bucket, and take UV light pretty good too. They are "double rimmed" but I use a skill saw to cut off the top rim to lower the top edge...loosing a few gallons of capacity.
I have since switched to stainless for a more professional appearance, but I used the cow buckets for probably 8 years...and the only time I had to replace them was when I had a dog that liked to chew for a year.
You can get them for free if you find someone that has cows.