I would recommend first trying to auger out or snake the bathtub drain to see if the clog will clear because it's unlikely that the drain will clear with the Drano if it's full of water because it will dilute the Drano too much to allow it to work.
You need to get the water out of the tub first and you can use a bucket to drain most of the water out of the tub and then use a shop vac to remove the rest of the water and then pour the Drano down the drain.