Well, I used to drop in powder that chlorinates the water AND that was 100% safe for even the smallest children (you know, they sometimes drink from it, whether you like it or not). But it couldn't help against the water turning green and smelly in no time when the (3 Muscovy) ducks arrived and started pooping in it. When they were small there was no problem, the water stayed pretty clear (most of their poo dropped straight to the bottom), until they grew a few weeks older. OH BOY! Talk about stinky stuff! Adding more of that powder was a no go and even heavy filtration couldn't get the job done. Now see, it's not the green algae that you should be worried about, though it's nasty, full of bacteria and I wouldn't recommend swimming in it

, it's the blue algae that can kill people AND animal life. This blue algae can bloom overnight, and if you haven't noticed it and take a dive in it, or you let your ducks swim in it... yeah, you'll find yourself in deep sh*t (no pun intended).
Anyway, with that being said, if you have a large pool and only a duck or two, by all means: go for it. But keep in mind that it WILL turn green, no matter what you do. If it's a very large pool, it might take some time, but eventually it will happen. Only doing regular full waterchanges and having a super large filtration system will keep you somewhat on top of it. And if you don't have a well or any other natural source of water at your disposal: it can become very pricey, trust me!
We got rid of our pool in total and gave our 3 ducks large kiddy pools to bath in. And they are happy with it. I myself have the luxury to live near the beach, and in winter time - when the sea is too cold obviously to even dip your little toe in - the local public swimming pool is around the corner.