Some questions about swimming's contribution to fitness

Hi, I am a new member here. Due to a serious knee injury (4 time dislocation), I am interested in taking up swimming. It is especially appealing as it is a low impact sport and involves all parts of the body. How good is swimming for the heart? I know it forces my heart to work, but is it good to keep it in shape? Is swimming good for strengthening a knee? Lastly, swimming is the only sport/exercise I do. As a result, I assume I would have to do quite a lot of swimming to make up for the lack of other exercise. How many hours a week would I have to put in (assuming I swim constant lengths with some rest in between - or as far as I can, but not just standing in the pool). Sorry for all the questions on my first post Thanks