The trouble with water is that it is a GREAT thirst quencher. Nothing is better for it. The problem is that since it so effectively quenches thirst, it often triggers the body's response to do so before we are adequately hydrated.
Yes it is. However we regularily consume more food than we are hungry for so consuming more water than we are thirsty for is not an issue when you know your life is on the line. And it is.
You should as stated start hydrating several days before a trip. In fact you should properly hydrate EVERY DAY even if you are not diving. You should consume half your body weight in oz's of water. So if you weigh 200 pounds then you need to consume 100 oz's of water. Take a 64oz juice or gatoraid jug and fill it with water. Then continue to drink it until its gone. Or count out enough bottles of water and do that.
Tips for Drinking Water:
1. Drink between meals. Your body captures the hydration more effectively, you will eliminate most minor hunger signals (you were really thristy but interpreted it as hunger) and you won't dilute the digestion of food.
2. Use travel time as 'water consumption time'. Or standing in line time or any other activity that would otherwise be wasted as a trigger to drink water. CARRY YOUR WATER WITH YOU and it will carry you!
3. If you MUST flavor it do it with NATURAL flavors from juice or lemon but NOT crystal light or fake/artificial ingredients.
4. Once you PURGE your body of toxins and all the chemicals that we dump into it...the desire for more chemicals/sugars/artificial sweetners subsides. Soon you ONLY crave WATER!
I would say if you can hydrate properly with water, then do so. If you can't force yourself to drink enough water (I sure can't) then something like Gatorade might be the better way to go.
If the choice is water with sugars (calories) and electrolites vs. no water then choose the former. But remember you will consume far more calories than needed and for most of us if diving is our only activity...we will be pretty hefty divers pretty soon.
Again the more you purge your body of unneeded chemicals/toxins etc the more your body will crave WATER. If you are an athalete (not a weekend warrior) but a real athalete then alternating between gatoraid and water is a good idea. The rest of us just want to FEEL like an athalete because we drink what they drink.