Constant resistance
When exercising in the water, there is a steady resistance pushing back against every mm of skin that is facing the direction in which you are moving.
This resistance causes muscles throughout the body to work just a little harder to move in the desired direction. Moreover, as the direction or movement changes the resistance remains consistent and balanced.
Cooling
When working out in the pool, the water temperature is lower than your body. This constant flow of cooler water helps to keep down your core temperature. This allows for a greater volume of work as your body will not overheat in the same way as it does on the land.
Constant need to balance and maintain position
When moving through water the muscles are constantly being challenged to hold the prescribed position and perform a specific movement pattern. Counter movements are often necessary to support the primary movement. Thus, virtually every exercise is a total body functional movement. Rarely is any water exercise a single joint exercise. The core is always engaged, the muscles around the joints are continuously being asked to contract to help stabilize the joints. Thus, water exercise provides a powerful full body workout.
Lack of gravity
Perhaps the most unique quality of deep water exercise is that the body is moving without ground forces pressing back through the joints and muscles. It is this quality which makes the water an ideal environment for joint friendly exercise at every level. From stretching to high level interval or power training, the non-gravity environment adds an entirely different modality that is helpful at every level of fitness.
The lack of gravity also provides an environment where the body can completely relax and be supported by the water pressure, eliminating the pressure on joints throughout the body. For backs, this often results in instant pain relief. For an injured knee it can be possible to move the joint through its entire range of motion without pain. This is a critical part of recovery as the surrounding muscles can re-learn how to contract without causing stress to the joint, speeding up the healing process.
Virtually anyone who is in search of improved fitness can benefit from deep water exercise. Whether out of shape or overweight, recovering from injury or seeking elite/professional sport fitness training, there is a water based program that, when added to your land routine will safely and very effectively bring you to higher levels of fitness than land only based exercise.