Water based exercise has the advantage that is non-impact. This makes it particularly suitable for anybody who suffers from arthritis, or who has problems with joint pain. Where running can cause long term pain, swimming would not.
The soothing property of water means it is effective at stress relief. This is why spa resorts are popular. Some say this feeling originates in the womb (who could tell) but the weightless feeling you experience when in water has a comforting effect on the psyche.