If the water is not wadable you should have a boat. Hunting small farm ponds where if a bird floats out of reach and you can walk around an get it when the wind blows it into range is acceptable, but larger bodies you need a boat.
IMHO it is border line unethical to hunt waterfowl where there's a good possibility of not retrieving a downed bird without the use of a boat. In some states like N.J. I've seen wardens watch some that didn't have boats where having a boat would be expected get fined for not including an unretrieved bird in thier limit and thus getting fined for over limit and "wanton waste"