My dad keeps his dentures in water all the time. He said it keeps them cleaner and they don't dry out.
He likes to use baking soda and vinegar to keep them clean. He's not a fan of the mint flavors in the commercial kinds, so he does his own. He usually gets a glass of water, adds a couple of capfuls of vinegar and then a couple of teaspoons of baking soda. When the soda dissolves, he puts in his dentures and soaks them overnight. He said the vinegar flavor is better than the fake mint in the kind you get from the store.