There are fiberglass repair kits out there for tubs, showers and surrounds, but I've never used them so I cant state if they actually work or not. I have seen a professional repair done to a shower though and you'd be hard pressed to find the repair. One question in my mind however is why did they crack to begin with? Someone slip and fall into a wall or are there settling issues with the house, etc.
I'd consider just replacing the walls. You can buy fiberglass surrounds for the tub or even replace the fiberglass with tile or marble walls. There may be other fixes but I dont think I'd want to worry about a leak through the shower walls messing up the ceiling below.