Here's a 2012 article by Laura Jones on the benefits of swimming.
www.usms.org/.../articledisplay.php
The Physical Benefits of Swimming
Lowers blood pressure
Reduces bad cholesterol and raises good cholesterol
Aids in weight loss and weight maintenance
Is easier on the joints than impact sports
... etc...
The Mental Benefits of Swimming
Enhances the connections between left and right hemispheres
Prevents brain shrinkage
Can help you make new brain cells
Improves problem solving skills and memory
... etc ...
The Emotional and Social Benefits of Swimming
All different levels can exercise in the same pool at the same time
There are toys such as kickboards and pull buoys and snorkels and fins to make a workout different and exciting and easier or harder as need be
It’s just plain fun!
... etc ...
If you want a visual illustration of the benefits of swimming, check out these guys, setting world records in their 90s! (http://tinyurl.com/pna9vyn).
As Tiger Holmes (93) put it "My doctor told me I'd be dead 10 years ago without swimming" :) A simple message that goes a long way!