SWIMMING.
Is easy on the joints.

Swimming, like walking, needs to be brisk to give real benefits.

It can be used to develop:-

Muscle strength.

Cardiovascular fitness.

Flexibility, for example in your shoulders.

It is also of great benefit to people who cannot carry their own weight.

To set up a routine you can talk to a coach or trainer or you can start in a gentle way and build up the duration and work content of your sessions as your strength and stamina increase. The main thing is to start!

Exercises done while standing in water, known as Aquaerobics, can help you to maintain balance. But whenever you are in water your weight is supported by it (by what is known as displacement) and as a result there are no benefits in terms of bone-density.

Research into Ageing's verdict;
Probably over-rated and needs to be complemented by 'weight-bearing' exercise for older people to maintain their bone-density.