Bone density problems

High impact activity increases the stimulus on your bones to increase their density.
The effects of this are most remarkable in adolescence, as our bodies are rapidly growing, but they occur throughout the lifespan.
Non weight bearing exercise does not cause the same improvements. It is important to include some level of cross training in your regime to avoid injury, but also remember that, even if swimming is your sport, the effects on your bones of getting out for a good walk a few times a week cannot be underestimated.