The common wisdom is that in the summertime, you'd better drink, drink, drink water to stay hydrated and healthy while your body is naturally shedding higher amounts of water to increased perspiration. It's not wrong thinking, and the effects of dehydration can be brutal: headaches, mental fogginess, sleeplessness and restlessness, dry mouth and skin, sunken eyes, muscle aches, rapid heartbeat, eventually shock, unconsciousness, and delirium. You can die from not having enough water in your body, which is, after all, composed of about 66% water.
We hear less often about the flipside of the same coin. You can, actually, die from drinking too much water.
Now, to be clear, you probably won't, but hyponatremia is ingesting too much water for your body, your kidneys, to be precise, to handle and process. The actual thing that will make you ill or can, in extreme cases, be fatal, is over-diluting the vital salts your system requires to survive. Too much water, too quickly, throws off the balance, and that's when trouble begins. "Water intoxication" can show up as headache, fatigue, nausea, vomiting, frequent urination, and disorientation. Guzzling a huge amount of water, even during exercise, is not always the best bet.
You've probably heard that the average adult needs to drink eight 8oz glasses of water per day for optimum health, but that 8x8 formula has been disproven under normal circumstances. Exercise increases water loss through sweat and usually increased respiration rate (heavy breathing--each time you exhale, water evaporates from your breath) so that water needs to be replaced, but not in a way that floods the system.
To counteract too much water, your cells swell and expand. That's one of the cool thing cells are built to do...however, in the unyielding armor-like cage of your skull, there's not a lot of room to balloon out. Think of how dangerous a concussion can be--when the brain swells against the pressure of the unmoving skull. SImilarly, waterlogged braincells press against bone, and that can manifest as seizures, coma, respiratory arrest, brain stem herniation, and death.
SO even if you're drenched from a summer run or bootcamp beach workout, drink to slake your thirst, and probably a little more (we are notoriously bad at recognizing thirst, mistaking it for hunger, being tired, just feeling crummy, or just ignoring it until we get cranky and foggy in the brain). You have to work pretty hard to overdrink water. Under normal circumstances (extreme athletes can have systemic shutdowns the rest of us don't face--so a marathoner has different rules), check the color of your urine each time you go--while we expect yellow, the lighter the better. More water in your system makes lighter urine--and just about colorless/clear is pretty great. Dark urine is a warning.
The delicate balance of our bodies is pretty phenomenal--just like eating--calories in should be balanced by the calories burned, and water in should be balanced by the water expelled. Don't overdo it in either direction...and whatever you do, don't drink your water from a single-use plastic bottle!