Mostly because I am both cheap and stubborn, I've probably "repaired" as many wading boots as anybody. Within reason, it can almost always be done. But you need to reconcile yourself to the fact that virtually any repair you make is going to be temporary and you'll have to do it again and again until you get tired of doing it and get another pair of boots. All the same, a well thought out and executed repair can last a couple hundred hours of wear time or more, depending on the extent of the original damage, etc.
So far as adhesives, I've had acceptable luck with: Aquaseal, Goop, various marine epoxies and even Gorilla Glue (mind its tendency to expand though..). I think David's suggestion to consider adding some new material to serve as a host/enhancement for the bond is real good advice. The other thing I would suggest is that whenever you repair a pair of shoes, always make a last or something to simulate the shoe in use in terms of shape. I generally use wads of newspaper stuffed to completely fill the shoe. This can make a big difference in the durability of your repair.
In the end though, once the original seams/bonds begin to fail, you're on the downhill side of the slope and the best you can do is delay final failure for a while. I've always enjoyed seeing how long I can keep them going though....
