Swimming is great all year 'round!
Breast stroke probably works the cycling leg muscles most closely, although the twisting motion of whip kick can aggravate existing knee problems.
Lower back pain usually means poor body position, such as swimming front crawl flat on the stomach and lifting the head up to the front instead of rotating the whole body about 30 degrees and swivelling the head to the side to breath. The body and neck should be straight and neutral at all times.