I have learned that it's nearly impossible for 100%. However, I have found if you put the water container, inside a larger containers, to catch the splash, it helps. My ducklings are 3 weeks old now.
Ducks need constant access to water, as it is how they keep their airholes on the bills from clogging. If those air hole become clogged, they could have breathing problems very quick.
I let my ducks out during the day when it's 50 and sunny. Then put them back in. It's advised not to let ducks go swimming, until they are at least 3 weeks old, as they don't have the waterproof oils yet on their body and could easily down or get sick.