I've had two knee replacements in the last year (left knee in May; right knee in November) and the best exercise I get is in the pool! In my early sessions (about 3-4 weeks post-op) I can manage about 20 minutes. Now, at 8 weeks post-op, I am up to 30 minutes in the pool with 5-8 minutes in the hot-tub (not too long, or the knee will really swell with the increase in circulation!!).
I try to go swim 2 to 3 x each week. Every recovery is different, but do make use of the advice your surgeon and physicial therapist offer to tailor your safest route to recovery!