Swimming is the perfect answer to your cardio problem. I swam on a team for 10 years and was in best shape of my life during that time. The resistance that the water provides is great for toning, but it's low impact so your not likely to hurt yourself further. Plus it has the added bonus of being a whole body work out.
As for why you never see anyone in the pool it's my opinion that people think you cannot get a good workout in the water. Most don't realize how hard it can be until they try. Also people just don't like to be wet and smell like chlorine. Especially in the winter time.
As for keeping up for stamina and metablism make sure you challenge yourself and let your workouts get harder. Your cardio won't benefit if you keep doing the same workout week in and week out. Goodluck!