Shouldn't the water level be at the outline of their shell, otherwise they can drown? I was told..
I don't know what you mean by "at the outline of their shell". The water needs to be deep enough that they can submerge their whole head when they dip it down, but not so deep that they have to reach their head up to keep their nostrils out of the water.
I do what Mike does. I make the water about half way up their shell. So if the tortoise is 2" tall while the plastron is resting on the ground, I use about an inch of water to soak them in.