told to swim, but hurts knees

hi, ? suffer from CM grade 2 and am doing PT for 4 month now. I have the same problem when I swim. After swimming my injured knee hurts for a day or two. I try to swim 2 days a week for a distance of 750 - 1000 meters freestyle and back swimming. My trainer makes me work my legs with the kickboard (4x50 m breast and 4x50 m back), although my legs are much stronger ? think still not strong enough and therefore ? have (and maybe also you ?) the knee pain. You can test your leg strength with a machine called cybex (avaiable mostly at university hospitals). You could also try cycling but ? have no experience with that because ?m waiting for the better weather. IN short ; you should train your legs and the only two things (beside surgery) you can do is swimming and cycling. What grade is your CM ?