When you sweat, your blood plasma volume decreases, which makes your heart work harder, increases your heart rate ("cardiac drift") and tanks your performance!
What is cardiac drift?
Cardiac drift is when your heart rate (in beats per minute) increases, or 'drifts' upwards, during a run of 30+ minutes, despite you staying in the same effort zone or pace race. This is more pronounced if running in hot and/or humid conditions, or when you are working at a very hard effort.
What causes cardiac drift?
Simply put, it's theorized that cardiac drift is caused by an increase in core body temperature and a decrease in blood plasma volume.
Core Body Temperature & Cardiac Drift
Your core body temperature increases during exercise as your body starts burning fuel for energy; the rise in your core temperature is a reflection of the increased activity occurring in your body at the cellular level. Cardiac drift will occur because of this rise in core temperature, even if the rise is very slight. In fact, you might not even notice cardiac drift on some runs because your core temperature rises only a little, especially in cool/cold conditions, because the weather also acts as a "check" and keeps your body cooler.
Sweating & Cardiac Drift
Sweating is your body's natural cooling response to this increase in core temperature, to ensure that your core temperature doesn't rise too high. When your body sweats, it uses the evaporative cooling effect (i.e. when sweat evaporates off your skin) to cool itself down. The warmer your core temperature (i.e. when you're working very hard or running in hot weather) the more your body will sweat to attempt to cool itself off, and sweating volume will increase. If running in very humid conditions, the evaporative effect is stymied because there's already so much moisture in the air, sweat can't evaporate and provide the cooling effect to your body.
Your body gets the moisture for sweat from your blood plasma.
Sweating & Blood Plasma Volume
Your body pulls the moisture it needs for sweat from your blood plasma, or the the yellowish "liquid part of the blood that carries cells and proteins throughout the body. It makes up about 55% of the body's total blood volume."  This means that the more you sweat without rehydrating, the more your blood plasma volume is reduced, 'concentrating' your blood. The heart must pump harder to move this 'thicker' blood throughout the circulatory system, causing a rise in heart rate beats per minute.
So what's the solution here? KEEP BLOOD PLASMA VOLUME CONSTANT to make it easier for your body to cool itself and prevent cardiac drift!
Hydration & Plasma Volume
When you sweat, you lose more than water. Sweat comprises :
This means that when rehydrating, water isn't sufficient - you must replace these lost electrolytes as well. Each person is unique, and "sweating rate (SR) and sweat electrolyte concentrations can vary considerably as a result of many within- and between-athlete factors."  This means there's not "one-size-fits-all" recommendation for hydration. but here are general guidelines to follow.
Rehydrating 101: Water AND Electrolytes
It's most important to replace lost water, sodium, and potassium during exercise, although some electrolyte-replacement products like the excellent SaltStick Caps also include calcium and magnesium.
A simple DIY solution is to add a pinch of pink salt to your water bottle or hydration pack (1/4 tsp of salt per 500mL/16 oz of water of water). Pink salt or other naturally occurring, unrefined salt like sea salt is preferable over iodized table salt because it contains trace minerals like... potassium, magnesium, and calcium!
How Much To Drink While Running?
This all depends on the conditions and how much you're sweating. Some people are heavy, salty sweaters; some merely glisten, even in the height of summer heat. In general, more body mass = more sweating, and harder effort = more sweating. If you REALLY want to know your personal 'sweat rate', by all means, use this sweat rate calculation protocol from the University of Connecticut.
IN GENERAL, aim to drink 3-6 oz (90-180mL) of hydration beverage (water + electrolytes) every 20 minutes of running, keeping in mind that most people can only absorb a MAXIMUM of 750 mL of beverage per hour, even in very hot conditions.  You will want to tailor this to your individual effort, body composition, and conditions; you may need more or less than this. MOST RUNNERS in MOST NORMAL CONDITIONS will be just fine if they drink 'to thirst', which is exactly what it sounds like: drink when your body tells you to.
It's entirely possible to drink too much water. Hyponatremia is a "decrease in serum sodium concentration caused by an excess of water relative to solute."  This happens when you drink too much PLAIN water without also replacing electrolytes, diluting the sodium in your blood to dangerously low levels. Hyponatremia symptoms include: nausea and vomiting, headache, confusion, drowsiness, irritability, muscle weakness/spasms/cramps, and seizures.
Mitigate the effects of cardiac drift by keeping your core temperature in check and your blood plasma volume topped up by staying hydrated with water AND electrolytes while running!