Plugs leak unless your drain is brand new with no little pits or bumps in it. If you don't have a tarp to keep the integrity, scour around the drain and duct tape over it. Then get a fruit can and fill it with a ziplock of gravel, rocks, or something as heavy as you can find and put it on top of the duct tape. This will effectively seal the drain and you'll keep all the water in the tub.