You must thoroughly drain the water and wipe the area dry, then you can get any kind of 2-part epoxy that is recommended for fiberglass. If they are real bad, you can get fiberglass resin and glass cloth from your local Hobby Store. If you get a 2-part epoxy, you may want to get a 15-minute or 30-minute epoxy like some of the ones made by Devcon. A faster setting epoxy will set-up quicker so if these crack are along the sides, it won't be all running and dripping down the sides. In this case you must work quickly and do small sections at a time. If you want to work a larger area, you can thin the mixed epoxy with rubbing alcohol and actually brush it on thin. Either way, I would recommend roughing up all the areas you will be sealing with some 120 grit sandpaper first. This way the epoxy will adhere good and it won't chip off after it hardens up. I highly recommend Devcon if you can find it because it dries totally clear. Once you have the tub filled with water, you probably won't even see the areas that you repaired.