By its very nature scuba is dehydrating, and has fluid reduction actions not shared by many other forms of recreation/exercise.
Looking first at peripheral dehydrating features, scuba trips often involve air flights, and these are notorious for causing fluid loss through both the skin & lungs. Commercial carrier cabin air can be drier than that on the world's most vicious deserts.
At the end of the flight, the diver often arrives at a tropical location, and this means exposure to the drying effects of air conditioning.
Outside of the AC facilities, s/he is exposed to high heat, humidity & sun, with ensuing fluid loss due to perspiration.
Tropical location, warm & sunny, and on a vacation? For many travelers this means a larger than usual number of cold beers & Jimmy Buffett boat drinks. The mild diuretic effect of alcohol is well known, and such fluid intake may result in the diver eschewing more appropriate liquids.
Turning to some more proximal contributors, super dry scuba cylinder gas (and often many tanks of it) has already been mentioned, but is only part of the story.
Don't underestimate the contribution of immersion diuresis. Topside, gravity causes blood to pool in the legs as we are sitting or standing. However, once the diver enters the water the effects of gravity are greatly attenuated. We are now subject to the atmospheric pressure of water, and this tends to move blood out of the legs and into general circulation.
Plus, the water is nearly always cooler than the ambient topside temperature. This causes the body to act to reduce heat loss with constriction of the vasculature near the skin and in the muscles of the extremities with resultant movement of blood to the core of the body, e.g., lungs, heart, major blood vessels.
These responses to immersion in turn result in the body sensing an excess of fluid, and through a hormonally-mediated response the kidneys are stimulated to produce increased amounts of urine. More fluid loss.
As for staying hydrated, the diver should begin hydrating several days before scuba, and continue throughout the duration. About three liters of fluid intake per day should be sufficient under most diving scenarios, although the normal diver is free to drink more if indicated. Fluid intake should be spread throughout the day, and not simply occur in heavy amounts just before or after a dive.
It is very difficult for the healthy individual to drink enough fluid to bring about water intoxication, although this condition can be quite serious & even fatal if it does occur. Alternating electrolyte-enriched sports drinks with water & juices will further reduce this already remote possibility.
The simplest way for the diver to gauge his or her level of hydration is to monitor urine output & color. It should be copious in amount and clear or very pale yellow in color. If urine is scanty or dark, fluid intake is very probably inadequate.
Hope you found this informative.
DocVikingo
