Yes you can use liquid plumber in your bathtub but you should remove as much water from the bathtub first with a bucket and then if you have a shop vac you can drain the rest of the water out.
Then you can pour the liquid plumber down into the bathtub drain and allow it to work on the clog.
If you have hair clogging the bathtub drain you can use vinegar and baking soda mixed together in the drain and then put the cover on the bathtub drain and let the vinegar and baking soda to work and dissolve the hair in the bathtub drain.