I agree with keith, you've got to deal with all the damage first, then you can worry about what you're going to do with the leaking surround.
If the drywall is wet, it needs to be removed, same goes for any insulation that may be affected. With the wall opened up, you can address any rot issues, if it penetrates less than 1/4", you can sc**** and treat the wood to prevent further rot. How is the floor/subfloor under the tub/shower? If water was getting into the wall enough to rot the framing, then it has likely gotten to the floor as well.
I would recommend consulting with local contractors with dryrot and bathroom repair experience. They can not only give you an idea as to how much damage there is, but the cost of repairs too.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!