Aqua aerobics, another form of aerobic activity provides the same training benefits as low impact aerobics done on land.  If you exercise at the same intensity in water as you do on land your lungs and heart will get as good a workout and with less wear and tear of you body. Because water is buoyant, aqua aerobics is non stressful on the body. Water that is waist to chest level supports about 50-75% of your body weight so it is much easier on your joints and muscles.

Despite its gentle cushioning, water provides twelve times more resistance than air. You can strengthen opposing muscle groups with just one exercise since water resists your movements in all directions.  Aqua aerobics benefits your entire body when done steadily and vigorously.  It increases aerobic endurance, improves flexibility, and tones the body as well.  

Your heart rate will be lower during water exercise than during land exercise.  Yet you will receive the same benefits.  A study showed that water based heart rate is 13% lower than land based minimum and maximum counts.  Still one would receive the same metabolic and Cardio benefits.  Therefore you can deduct approximately 13% of your target heart rate range while exercising in the water.  There are various reasons why heart rate is, lower during water exercise.

Heat

The body creates excess heat during exercise.  Because water dissipates heat more effectively than air it is less stressful for the body to get rid of excess heat in the water hence resulting in lower heart rate. 

Gravity

Water lessens the effect of gravity on the body.  Not only is it easier to spring high in the water it is easier for your blood to pump uphill back to your heart

Partial Pressure

Oxygen may enter then blood stream more readily due to water pressure thus reducing stress on the heart and providing oxygen to working muscles.