To Roger's point...
There is a product called Tear-Aid. There are two types, A and B. I thought I might have first heard about it here, but a search came up empty. I got some to have in my repair kit for my dry suit. I haven't used it yet, but remember reading that several folks, on some board, rave about it.
You can request samples here...
http://www.tear-aid.com/
and you can purchase it at Amazon and Cabelas.
Depending on where and how big the tear is, it might be worth a try.