Sorry to re-post - all the carriage returns were stripped. I forgot to add HTML. It reads better with some whitespace.
I'm going to try to to untangle some if the above...
Some guy somewhere once said that you need to drink 8 glasses of water a day to maintain health. Turns out to be nonsense. For those of us with intact thirst centers in the brain (pretty much everyone reading this, for example), drinking when you are thirsty is all you need. Your body will tell you when you need more liquid by using the thirst mechanism. There are exceptions, as there is a lag before thirst is triggered, so on a hot day when you're exercising aggressively, you can get "dehydrated" and not get thirsty in time to do anything about it, but this is rare, and recent evidence tells us that hydrating aggressively, even in marathons, is overkill.
The jumble of hyponatremia, hypernatremia, hypertonic, hypotonic, hypovolemia, hypervolemia, isotonia, etc is maybe worth clearing up, although this will be interesting to precisely nobody. Some of the concepts are almost right.
"hyper" = too much, "hypo" = too little, "iso" = equal.
"volume" is the quantity of fluid (any fluid, technically) in the system.
"natr*" = sodium in the system.
"-emia" = in the blood.
"tonia" = concentration.
So, hypovolemia = low volume of fluid in the blood (hypo, vol, emia) Isotonia = equal concentration (in the medical context, meaning concentrations of a solute equivalent to those found naturally in blood). If you drink excessive fluids over an extended time, you overwhelm the kidneys' ability to maintain normal sodium concentrations in the blood, and you end up with hyponatremia. Drinking excessive fluids is usually called "psychogenic polydipsia," which is med-speak for drinking too much water because your brain is bad. The hyponatremia is potentially fatal, and often causes confusion, among other symptoms. Note that it does not cause (at least immediately) hypovolemia - the quantity of fluid in the system is adequate or high, it's the composition of that fluid that's troublesome.
In this case, one could say that the composition of the blood is hypotonic - there are fewer solutes (particularly sodium) in the blood than normal. This is treated by limiting fluids (reducing solvent, and allowing the kidneys to recover and restore balance), &/or by increasing sodium intake. Pepperoni pizza is a great solution (not kidding - my favorite nephrology professor used to prescribe exactly that). Hypertonic saline is reserved for emergencies. The blood is usually about 0.9% sodium, so a 3x concentrated version of that - typically 3% saline - can be given parenterally (via IV [intravenous], for those of us scoring at home). This is a dangerous treatment, as the brain is susceptible to dangerous/fatal swelling if hyponatremia is corrected too quickly ("cerebropontine myelinolysis," if I remember correctly - I'm a cardiologist, and I haven't thought about this stuff in a long time).
Not drinking enough fluid results in hypovolemia (commonly called "dehydration"). Usually the sodium levels in the blood measure high ("hypernatremia"), although it's not due to too much solute - it's due to too little solvent. The treatment is to replete fluids (volume), either with a a straw and some water, or with IV hydration. Usually 1/2 NS (saline that's hypotonic compared to normal blood, in this case 0.45%) or even normal (isotonic, 0.9%) saline.
The rest of the parent's post is mostly on target. Sorry for pedantry.