If the crack goes ALL the way through, I think you can pretty much forget it. I can't tell from here but that was the general scuttle. Why (you ask?) Apparently, a repair won't hold since the repair-material isn't 'structural'. Even if you could support the crack and fix it, over time, it will break again. I have read that it MAY be possible to inject an Expansion Foam underneath which, when dry, may help support the tub. But, I'm not sure if, over time, the foam gets 'compressed' and useless in terms of support. Talk to a Pro who KNOWS what they're doing.
I guess I'd have different advice if the crack wasn't in such a 'lethal' location in the tub (like up on the side, or something.) But where it is, it's going to experience a LOT of stress as time goes on.
If you end up ripping the tub/shower out, be sure to replace any rotted wood. Also, be sure there is support underneath (for the replacement tub.) This will add 'life' to the new tub. Take pictures of the 'job' as you go for your records too. You never know when you may need to refer to them (like if it leaks and you want to know which direction the pipe runs.)
My best to ya and hope this helps.
Jay J -Moderator
PS: There are 2 types of fiberglass - polyethylene based and epoxy based. I don't think the repair materials work well interchangably. Be sure to find out which type you have. I know the repair stuff stinks so be sure to open some windows on the day you intend to attempt the 'fix' ...