Immersion, along with a water temperature that is colder than the air, causes narrowing of the blood vessels in the arms, legs, hand, and feet. This vasoconstriction occurs primarily in the skin and the superficial tissues of the body, but also occurs in the large muscles of the arms and legs.
The net result: A decreased blood supply in the skin and the muscles of the extremities. However your heart continues the pump the same amount of blood which causes an increased volume of blood to be sent to the central organs of the body such as the heart, lungs, and large internal blood vessels.
The hormone that controls the production on urine by the kidneys is called the anti-diuretic hormone (ADH). It is this hormone that controls when and how much urine your kidneys produce. The increased blood volume to the major vessel is interpreted by the body as a fluid overload. This overload causes ADH production to stop, which in turn allows the kidneys to immediately produce urine to lower the centrally circulating blood volume - the body's automatic response to regulate the amount of fluid or blood volume.
The problem is that, once we've fooled our bodies into thinking that we have lots and lots of fluid, we get out of the water. Then the circulating blood volume returns to near normal - less the fluid taken to produce the urine and that fluid that has been voided. The body takes steps to quickly replace the lost fluid and draws from body tissues. This action causes the body to sense a low volume and it craves fluids. So, you drink a bunch of water, which makes the problem even worse on the next dive.
Just pee before the dive and you should be ok!~