First, get an exam to see if a deviated septum is indeed the problem. Even if your nasal passes are partially blocked this should not greatly hurt your singing if you keep your throat open and use good breathing techniques. Blocked nasal openings will not cause a nasal sound if you use the above techniques. You can always breathe through your mouth. So yes, you can sing very well with a partially blocked nose. Practice inhaling with the feel of the beginning of a yawn. This opens the back of the throat so what you then need to do is to KEEP it open while you sing. Practice, practice, practice this, and then do it some more. KEEP THE BACK OF YOUR THROAT OPEN WHILE YOU SING. (This goes for all singers)
Whatever to recommend surgery for a severe deviated septum is a constant source of discussion among singing teachers. Most teachers admit that straightening the deviated (bent or crooked) septum (the bone and cartilage that separates the left and right nostrils) will make breathing easier and improve some important higher harmonics. There are differences in opinion concerning how much improvement will occur after surgery.
Years ago a student asked me for an opinion about his having the surgery for this problem. I did not know the answer so I called my mentor, David Kyle in Seattle. David was the teacher of such singers as Geoff Tate and Ann and Nancy Wilson. His answer was to definitely recommend the surgery, and so I did. The student showed improvement in various areas of singing afterword. Over the years I've worked with other students who had the surgery and in most cases there was noticeable improvement in their singing. I often recommend this surgery for extreme cases, while also making the singer aware of the other contrasting expert opinions. The choice is, of course, always their.
It's an uncomfortable operation with a short recovery time, and I can only relate my personal experiences concerning this topic. There's a lot of info to be found on the web. I recommend doing your research.