How deep is shallow?
It only has to cover the roof to drown a person inside.
If they were outside, their head would probably be above water as they stood on the bottom.
...typically, stock tanks in Texas are shallow (i.e. 2-5 ft). Right now nearly all of them are shallow or dry due to a severe, prolonged drought.
But, the wealthy owner of this little piece of ground was apparently keeping his tanks full using well water.
So, the tank could have been deep except that I read that deputies were standing on the roof but their upper bodies were out of the water.