I use a 60 gal stock tank. It it is half as tall as a normal stock tank and most of ducks cannot touch the bottom when it is first full. After the play and splash in it then most can brush the bottom with their feet. Before I got the stock tank we used a kiddie pool and my ducks were happy in that as well and would try to dive. Now they really can dive and do a lap or two before coming back up. I don't think it really makes all that big of a difference for them on how deep it is. My ducks go nuts in the little puddles that form in the yard, cement mixing pans, water bowls. I find the key is fresh water. They will go to any container that has fresh was vs a deeper pond with older water. Mine all will sleep in the water during the day too.