timmmay:
The WCC / Tank guy tanks aren't sealed, they have a hole near the top and the lid is just sitting there. I'd go for a barrel with a good screw on lid, but if you can't find one that WCC one is good enough.
The hole near the top is for the rainwater diverter spout to come in. The lid does lock on with a twist, but you are right it doesn't seal tight.
We had one at our old place and I was always surprised at how gunge-free the water was when I checked it, but there were definitely some bits of detritus floating in the water. You'd want to filter (and boil) it before drinking.