If its cracked and you want to seal it (probably around the drain poinr) see if there is anyway you can get to access the void underneath (you need a hole about 5mm diameter) a good route is up through the floor close to the shower drain where it comes downn through it.!!. If you can then get yourself a can of builders foam! It expands enormously, is exceedingly
sticky, and best of all? Its waterproof so will seal any crack.
I did exactly that with a MH shower tray that had cracked around the drain. It lasted for the next 4 years I owned the MH
If its a very small crack you might get away with Captain Tolleys Creeping Crack Cure