Oh, they love to make it dirty..... I use a variety of things. Lids from the Rubermaid containers, I cut doors out of and turn upside down for nest boxes. I had some thrift store used rubber mats, left over plastic floor from a shed floor. ummm, 4-6 inches of River rock. .....concrete stepping stones...... Mind you , they then try to carry it in still.... And some of that 'dirt' is poo anyway. Depending on the size of your pool and how many birds use it and if it is muddy in the area.... you will need to dump it every 2-5 days. I have a lot of birds coming and going, many are ducks who love to carry their food and drop it in too, and I usually need to to it every other day for smaller bins, and Runner duck water.And for the larger kiddie pools at least once a week.
Also depends on what you mean by "Clean". I always have clean drinking water in buckets, but they prefer the gunkwater they swim it often......sheeesh....
Concrete ......has a reason.....