Epoxy Resin. Mix a bit of Talc in it if the bath is white. Unless you get white epoxy resin that is.
Make sure the area is clean & dry. Rub a bit of wire wool or scouerer in the hole. Sticky tape behind hole to stop it running through. Only use enough to fill hole don't go mad with it.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix baths and showers.