Is swimming good exercise for and injured athlete and how much to swim?

[Twister]

I'm 28 and athletic, I compete in submission grappling/wrestling tournaments and recently herniated a couple of discs in my lower back. Ive gain 13 pounds in 2 months not training and seeing doctors and just found out whats wrong. The doctor said I could swim, I've only been an average swimmer, is swimming good exercise if your injured and also considering what I've said how much should I start out doing. I want a good work out but I really never trained as a swimmer before thanks