You can use mastic but in my experience it is only a short term soloution that will not last long. You can also have a more permanant repair by applying a fiberglass patch to the under side of the damaged area aprox 100mm (4") out from the split. This will stop the bath from flexing too much around the vunerable area of the damage while it is in use and under stress. Then sealing the split with an enamel repiar kit from you local plumbers yard available in the most popular colours and if done properly an almost invisible repair.