Maybe they know something we don't? Perhaps the dirt has the minerals and whatnot and it's more natural for them to drink from water with dirt under it, like ponds, rivers, etc.?
Finding ceramic bowls that big would be costly. But I'll take a gander anyhow. Doesn't cost to look! Wouldn't they crack in the winter when there's a foot of snow and sub zero temps?
We use a kiddie pool for our duck pond but have to dump it once a week minimum for them. Wish I could find a way to not have to do that! What a chore that is.