I am a student at ballet school and we have just finished studying a module on the benefits of cardiovascular fitness in dancers. We discussed that although dancers are often very anaerobically fit, they tend to be less aerobically fit. Therefore, we timetabled ways to fit in aerobic exercise around our dance training. We were told that swimming is the best way to do this as it is non-weight bearing, thus we are unlikely to injury ourselves, and it does not over develop any muscle groups in particular (if one was to swim a lot daily, then the shoulder muscles would over develop, but with our full dancing timetable, it is unlikely that we would have the energy!!)
Also, one of the other ballet schools in London, begins each day of classes with 1 1/2 hours of aerobics, in an attempt to increase the students cardiovascular fitness.