You know I have raised MANY MANY chicks and never had one drown...never used rocks or marbles in the water at all either. Just used a standard waterer and let them be. Never one accident. Now Seramas I do use the marbles for as they are SO tiny! But I would not worry about them drowning. They are bigger chicks and should do just fine.
i have a billion gazilllion post offices near our house, so which one will be receiving the chicks? the closest one?