• The DCS factor. Experts aren't sure why, but dehydration seems to increase your risk of decompression sickness (DCS). The docs theorize that your body doesn't off-gas nitrogen as efficiently when it's dehydrated. Whatever the reason, dehydration is recognized as a contributing factor in many cases of DCS—and it's one you can control.
• The fatigue factor. Dehydration causes fatigue. Muscles don't work as efficiently when you are a quart or more dehydrated and you get tired sooner. At a water loss equivalent to 3 percent of body weight (i.e., 2 quarts for a 150-pound diver), your muscular endurance diminishes.

The heart factor. Dehydration also causes cardiovascular stress. At four percent water loss, your heart must work harder to make up for a lower blood volume. That leaves it with less reserve capacity for dealing with the stress of, say, a long surface swim against a current.

That's a gallon a day. That's four quarts. Eight pints. Sixteen cups or, if you like, 128 ounces of water. If that sounds like too much, consider:

Now Go Diving
A day of diving can almost double your normal water loss. Here's how:

• Having fun is hard work. You work harder than normal, hefting tanks and weights, struggling into neoprene and finning through the water. So you sweat more and breathe harder.
• You have to "go" more often. In the water, two processes—immersion diuresis and your body's reaction to cold—both trigger the urge to pee, so you lose more water through your bladder than normal. In response to cold, your body constricts blood vessels in your arms and legs, sending it back to your trunk in order to conserve heat. For reasons scientists haven't quite figured out, simply going under water causes the same reaction. In both cases, your body believes its total fluid volume is now too high, and orders urine to be made and dumped to restore the normal level.
• You breathe dry compressed air. You breathe dry compressed air, which your lungs moisten with water drawn from your body's stores. So you lose more water from respiration than normal.

The result? By the end of the diving day you can easily be down a quart or two of fluids.
• Add travel, tropical climates, rum punch and repeat six days ... Air travel means the exercise of lugging luggage, plus hours of breathing dry airplane air. Your tropical destination includes more heat than you're used to (more sweating), and perhaps more air-conditioning—which dries the air. If you're unlucky it may include a bout of tourist's diarrhea, which, like vomiting, causes major water loss. Then there's alcohol in the rum punch and the caffeine in the coffee and sodas.

Dive as often as you can in this environment, day after day, and you can easily build up serious dehydration by the end of the week. Add accumulating fatigue from other causes like thermal stress, breathing against resistance and unaccustomed exercise and you could be headed for a train wreck.

• Drink water before coffee or alcohol. Think of the water as the admission fee before you can enjoy that cup of coffee or glass of beer. Drink one glass of water per beer, mixed drink or cup of coffee.
• Flavor the water. To make water more palatable, mix 50-50 (or weaker) with fruit juice. Or, if you don't mind artificial sweeteners, mix with a small amount of Crystal Lite or Kool-Aid. Add just enough of the powder to make it interesting—a strong solution may actually interfere with water absorption.
• Chill the water. Add ice if you can get it and trust it. (Sometimes ice is made from tap water.) No, you don't absorb cold water faster than warm, but cold numbs your taste buds when water has an unpleasant taste.
• Eat it. Most foods contain more water than you might think. For example, celery is 95 percent water, mushrooms are 92 percent and apples are 84 percent. Even a baked potato is 71 percent water. In general, fruits and vegetables have the most water, as well as the complex carbohydrates that are your best energy source.

Sports Drinks — Better Than Water?
Maybe a little. Research indicates that a small amount of glucose in the water increases the rate at which your intestines absorb it. Likewise, electrolytes, including a little salt, maintain the proper chemical balance in your bodily fluids. Hence Gatorade and its many imitators. These sports drinks also have a taste designed to not quench your thirst quickly—so you want to keep drinking longer.
But these sports drinks are really designed for rapid hydration following massive dehydration—for a football player leaving the field after a hot 10 minutes of play, for example. A scuba diver, however, faces gradual but steady dehydration, and needs gradual, steady hydration. Unless you're on a restricted diet, you'll get plenty of glucose and electrolytes from food and don't need to add any to your water. Too much glucose, by the way, interferes with hydration by slowing the rate at which water passes from your stomach to your intestines. That's why sodas and fruit juice are not as efficient as water unless diluted.

Why You Get Cottonmouth After Diving
The air in your scuba tank is as dry as the Sahara, the water having been removed from it in order to protect the inside of your tank and regulator from corrosion. Each incoming breath of dry air picks up moisture from your throat and lungs and carries it out when you exhale, so every breath represents a loss of water.
How much? Not as much as you'd guess from the cotton-mouth feeling that results. On a four-tank day you'll breathe about 260 cubic feet of dry air (assuming 80-cubic-foot tanks—actually 77.4—and a minimum pressure of 500 psi). The maximum amount of water that much air could have contained (at 100 percent humidity, 86° F) is about one cup. In fact, you probably lose less. Research reported in the journal Respiration Physiology suggests that most of this water loss occurs in the first 15 minutes when your upper respiratory tract dries out. Regulators that trap moisture in the second stage to moisten incoming air lessen, but probably don't entirely prevent, the drying effect of breathing tank air.

Hydration Hazards: Caffeine and Alcohol
Yes, a cup of coffee and a beer are mostly water. But the caffeine in one and the alcohol in the other are diuretic: they suck water out of your tissues and send it to your bladder. As a result, drinking 12 ounces of coffee or beer results in 12, 15, 20 or more ounces of urine and a net loss in your hydration score.
The sneakiest is caffeine. It's in more foods and beverages than you might think. Popular soft drinks have a range of caffeine concentrations between 36 and 48 milligrams, with Jolt Cola the chart topper at 72. Coffee has the highest concentration, up to 180 milligrams per five-ounce cup. Tea can have as much as 50 milligrams per cup, and both tea and chocolate are loaded with substances called xanthines, which are also diuretic.

Are You Dehydrated? Here's How To Tell...Sort Of
1) You're Thirsty.
Unfortunately, your sense of thirst isn't a reliable indicator of dehydration. It's what economists call a "trailing indicator"—it kicks in late and leaves you playing catch-up unless you start drinking before you feel the need. Bottom line: drink water when you're thirsty—and when you're not.

2) Check Your Urine.
Its color, amount and frequency will change if you are dehydrated. If you're drinking enough water, it will be pale yellow or clear (Nancy Clark, the New York Times nutrition writer, says your urine should be the color of lemonade, not apple juice), of your normal volume, and you'll feel the urge every two to four hours. Darker urine, less of it and the need to go less often means you're dehydrated.

Unfortunately, urine—like thirst—can lie. Diuretics like alcohol and caffeine rob water from your tissues and dump it in your urine, diluting it so it looks clear. Judging by the clear, copious urine which results, you may think you're well-hydrated, but in reality your tissues are even more dehydrated than before.

Dehydration and Medication
Many medications, both prescription and over-the-counter, can have a diuretic side-effect—causing you to urinate more and thus increasing the dehydration problem. Antihistamines including common cold remedies, are one example. (Fortunately, the ever-popular Sudafed is not diuretic.) For over-the-counter medications, look for a warning of dry mouth on the label. For prescription medications, ask your doctor.

Nine Ways to Beat Dehydration

1. Drink water.
2. Stay out of the sun and wear a hat.
3. Drink water.
4. Eat foods high in water, like fruits and salads.
5. Drink water.
6. Don't eat too much protein, the digestion of which requires more water than it contributes.
7. Drink water.
8. Take it easy with caffeine and alcohol.
9. And drink water. At least a gallon a day.