Go to a marine supply store (hardware store may have) and purchase Marine-tex epoxy in white (if your tub is white. Mask off the area around the holes and the bottom of the tub. Cut a small rectanguler piece of plywood the slightly smaller than the width of the hole and about twice as long. Screw a drywall screw in towards one end. Mix the epoxy as directed, butter the epoxy onto the plwyood, slip it in to the hole and pull it to the tub. do both holes...work fast...tie the screws together with wire. When it sets up, shave it off with a sharp wood chisel. Mix up a little more and fill the holes flush. Shave it off or sand it...do not sand tub.
The color will not match. But it will be water tight.
Or you could epoxy in an acrylic rod and have a washcloth holder again.