This is in my area (physiology, biology, clinical laboratory medicine, endurance athletics), so I have to join this thread. I'll make this as concise as I can. I'll elaborate if anyone wants me to.
Water is NOT by itself the absolute best hydrating drink. It is what hydrates you, but you also need electrolytes - sodium first, then less so potassium (but definitely both) - and carbohydrates. Exactly how much of each, and if you even need carbs (simple sugars or more complex carbs) in your drink vs. in a meal, is directly related to what exactly you're doing and how much you're losing fluids (i.e. sweat or breathing on scuba).
Sugar is not a diuretic. That web site stretches the definition a bit too much. It is, however, a "renal threshold" substance. More on that if you really want it.
Water is what hydrates, sodium and potassium (K) are the electrolytes you sweat out (with little bits of Mg and others). Fact is, water hydrates most effectively - even independently of all other factors - when it comes with a dash of sodium. If you're exercising and sweating, then the sodium, first, and K, second, become just as important as the water. Ask a GI doc about Na-coupled pumps in the intestines.
By "what exactly you're doing..." I mean how intense is your activity and just how fast are you (your muscles) burning fuel. So, this is where SubMariner and I go to the goo: intense activity (running/riding) and only if we're going for much more than an hour (PaulB). Sustained, intense activity (running, riding, hiking) burn through your fuel most quickly, which is why Gatorade and Cytomax (one of my preferences) exist.
Okay, so science of it all aside, we're doing scuba diving, which within this whole scheme is more water/fluid depleting (relative to electrolytes and relative to energy burning) than intense exercise. So, it's more a matter of rehydration/maintaining hydration than maintaining energy (read: carbs/sugars) and electrolyte levels. Of course, this isn't fixed; you have to ask yourself how much you're exerting yourself above and under water.
You will hydrate most effectively with water with some electrolytes and very small amounts of carbs (sure, okay, some form of sugar, though best if it's not the simple sugars you find in Gatorade). The easiest way to get these things is to dilute (to at least one-half, if not further) sports drinks. If your diving is not intense in your exertion level, then you will do best with regular food and good ol' water during your SIs. If you're trying to rehydrate fast - imagine a medical situation where the paramedics throw an IV into you, which will be of saline, plain and simple, highly diluted salt water, 0.9% to be exact - then highly diluted sports drink is best. Suggestion: avoid the ones with simple sugars (e.g. Gatorade) and go for the ones with more complex-carb versions.
Personally, I use a Hammer Nutrition powder called Sustained Energy, diluted to about one-fourth of their recommendation, and I never run into dehydration or cramping problems in any of my long-distance training (marathons, cycling) or when I'm diving. Never. As much as anything that means I've found what works for me. Nevertheless, rules of physiology don't change. I guzzle that one-fourth mix during surface intervals as much as I can - key: more than you think, and yes, dandydon is right, if you're rehydrating well you should be peeing lots, and it should be pale yellow, and yes, some electrolytes do come out with, because you never pee pure water....
Drink just water alone and you'll pee even more and hydrate less effectively.
Again, you simply don't need the simple sugars of Gatorade unless you're swimming really, really hard. Go for something more complex. Every athlete and diver needs to work out what formulation/drink works best for him/her. Just be sure it's dilute. My suggestion to people is that they start with water and add bits and bits of powder until it's palatable, instead of the other way around (progressive concentration instead of dilution). Make sense?
Budgy, you're (or rather the reference you cite is) correct; the short term action is as stated. But that's not a problem unless you're drinking concentrated electrolytes (see above.) The extra- and intracellular compartments will, under normal circumstances and if you're healthy, etc. equilibrate very quickly.
