My hydration prep begins the night before the game.. I will buy a 6 pack of gatorade, refrigerate 2-3 and freeze the rest.
I will also refrigerate and freeze a split of maybe 6 or 7L of water.
All this I bring to the field in a cooler, plus sandwiches and granola bars. Sandwiches if you have time to sit down, if you're not feeling up to it, snack on granola bars... 3 or 4 over the day gets me through. Not the super dry ones, but the sort of gooey ones.
Game day for fluids, I will carry up to 3L of water on my vest depending on type of game, though recently it's dropped to 1.5-2L cause I've been playing shorter games. I will take 1 bottle of liquid gatorade in a pouch... This is mainly to back up the water. Sometimes you get turned off the taste of water even though you need it.. you just can't get it down. A few sips of gatorade kind of reset my palate to let me get back on the water. I sip whenever I can, and I try to force myself to remember and take a sip even when I don't feel like it. One, like m102404 said, and two, you don't know when you will get another opportunity. You could be hugging the dirt for a few hours.
On skirmish I tend to go lighter, so 1-1.5L of water, 1 bottle. Leave the cooler at the safezone. In milsim, I bring the cooler onto the field to wherever the team camp is so I can refill. Depending on how I'm feeling after the first bottle of gatorade, I will pick between frozen and liquid, overtime it unfreezes as I carry it, guaranteeing that I have a really cold beverage when I'm really dying. I brought myself back from a really bad state by having it ready either on me or in the cooler.
You should always be sweating. Not sweating means you're on a dangerous threshold.
Also if you feel nauseous it's a super bad sign.
In the most extreme cases of lots of sweating/loss of electrolytes, gatorade will not replenish all of them. Basic sports drinks only have salt, poweraide has a bit of potassium.
You also need magnesium and calcium... in extreme cases, I have supplement tablets for the rest, though those jelly beans mentioned before might be tastier. Cramping and muscle twitching are signs that you are either dehydrated or low on electrolytes, though be careful when supplementing you CAN have too much. Same with water, too much at once will just flush through your body and not be absorbed. I had a bad experience with that as well, that's why sipping over time is a good thing.
I always bring way more fluids than I think I'll need. Better to go home with unopened bottles than run out midday. Most I ever consumed I think was 6L of water and 3L of gatorade in a 24 hour game. It was hooooooot.