There's always a possibility that chicks with very watery poop have something wrong going on, but if chicks are otherwise acting healthy and normal, watery poop just may be their "style".
I learned this last summer when I got four SS day-olds. Two of them had poop so watery that it was more like water than poop. I immediately thought that I was doing something wrong. I changed their water, adjusted the heat lamp, cleaned out the brooder real well, and fed them yogurt. Nothing changed.
Almost one year later, those same two SS pullets still have watery poop, are very healthy and lay every day. In fact one of them makes such a "splorting" sound when she goes that I can tell it's her without even looking.
I've never read or heard anything about watery poop being the normal "default" poop for some chickens, but it appears that my two are just such chickens. Perhaps there are more out there than we think.