First, get tested to see if a deviated septum is really the problem. Even if your nasal passages are partially blocked, this shouldn’t miss your singing too much if you keep your throat open and use good breathing techniques. Blocked nostrils 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 sensation of the beginning of a yawn. This opens the back of your throat, so what you need to do is HOLD it open while singing. Practice, practice, practice this and then do it some more. KEEP THE BACK OF YOUR THROAT OPEN WHILE SINGING. (This goes for all singers)
The recommendation of 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 upper harmonics. There are differences of opinion regarding the improvement that will occur after surgery.
Years ago, a student asked me for an opinion on his operation for this problem. I didn’t know the answer, so I called my mentor, David Kyle in Seattle. David was the teacher of singers like Geoff Tate and Ann and Nancy Wilson. His response was to definitely recommend surgery, and I did. The student showed improvement in several areas of the epilogue song. Over the years, I have worked with other students who underwent surgery and, in most cases, there was a 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 yours.
It is an uncomfortable operation with a short recovery time and I can only relate my personal experiences on this subject. Much information can be found on the web. I recommend doing your research.