Water temperature above 90o F will actually prevent the aquacisers from cooling off and cause an increase in their core body temperature. When submerged in water, sweat does not evaporate as it does in the air. It is this evaporation mechanism which cools the body as we sweat. Being submerged in water subverts this important cooling process. Therefore, water temperature should be maintained in a range that is several degrees cooler than skin surface temperature in order to cool the body. Skin temperature is around 93.5o F, which is why the cooling effects of water to the body ceases above 90o F, at least during exercise.
One of the properties of water is its effects on acceleration of the body. Since water is more dense than air, it has more resistance and it is harder for the body to accelerate through it. One of the factors modifying acceleration is drag - which is a force resisting the body's movement in water. Components of drag include small waves (called eddys) that ripple along side the body as it moves through water. the faster the movement, the larger the eddy, and the more resistance occurs. The second component is friction of the water to the skin, which adheres to skin by virtue of it being in the water. Again, as movement increases, friction increases. The third component is tail suction, which describes the inability of water to not fill in behind body parts that are not aqua-dynamic enough, thus the body must "pull along" a certain amount of water as it moves - adding to the resistance. Water surrounds the body and resistance is provided equally in all directions. The 3-D freedom of movement afforded in the aquatic medium allows the joints to move in a manner which is most natural for them. This means that water is not only non-injurious to the joints but when used properly, water exercise can potentially be therapeutic to the joints.
As any weight lifter or strength coach knows, there are two types of resistance, positive and negative. Positive resistance is described when the muscle contracts and overcomes the weight, lifting it off the floor. A negative resistance is described when the weight overcomes the contractual force of the muscle and the weight is lowered back down.
Negative resistance has certain advantages and disadvantages from a training perspective. "Negatives" as they are often called, play a major role in increasing the size of the muscle and tendons. The major disadvantage negatives present is that they also cause minute tears in the connective tissue which often result in delayed onset muscle soreness (DOMS) and the possible development of adhesions over time. Soft tissue swelling and a restriction of blood flow to the affected area are also a result of negative resistance.
Positive resistance has none of these disadvantages. When using positive only resistance, the next day soreness is greatly reduced or eliminated. Swelling and adhesions are much less likely to occur. From a safety standpoint, positive only resistance is vastly superior since the danger of a falling weight or rubber band recoiling is non-existent. These factors or comfort and safety, along with the increase in blood flow resulting from positive only resistance exercise, make it the preferred form of resistance for most forms of physical therapy and as well as general population conditioning.
Water is a positive only resistance medium. When moving an object through water, once the muscle ceases to contract, the water does not apply force against the muscle to force it back. Water merely resists movement. This makes it a passive resistance medium. As such, it allows the exerciser to be maximally aggressive. This liberation from the need to protect against the injurious potential associated with negative resistance, can be a very dynamic training tool for elite and professional athletes.