When they get tired they will not have the sense to get out of the water, if they get too tired they will waterlogged as they are not waterproof, then they can drown.
The safest way is to watch them the whole time they have access to it.
i'v never had that problem my ducklings get in and out by themselves and they were 1.5 weeks old. mine are mallards and cayguas, and i'm hoping to add some moscovys here soon as well
mine just sit and play in it occasionally. its a small kitten litter pan probably 2 inches deep and that's it they love getting in and searching the water for food. they stay out of it more than in it