This is the major drawback to having ducks. We have minimized it by having a child's pool and never filling it over 1/2 to 1/3. You will have to put a "stepping stone" in it so they can get out. If you put a sand layer around it, that will also help. The pond can also be elevated on wood pallets which extend beyond the pond and stop a lot of the dabbling. You should clean the pool regularly, so it is important to be able to transfer the dirty water out of that area. This can be facilitated by having a on/off drain in the pond with a long hose to remove the water. A more drastic way is to dig a  pit to drain the wetted area. roof it with pallets extending to the wet area and let the splashing drain away. French drains are another way. All in all, the hope is to limit how far it gets wet. In the rainy season, not much will help. If they are penned at night, use containers which are deep bedded and very shallow, just enough water to dip the beak. Fewer ducks mean less mess also. Good luck.