When you drink a lot of water, you are very hydrated. There's a lot that the question needs to be more specific about. Is it hot out? Is the person exercising? If the answer is yes to both, then a person is going to drink a lot of water throughout the day in order to replenish lost water.
If a person is not drinking the water to replenish lost fluids and drinks "a lot of water," then it is a question of whether or not the kidneys can keep up with the intake of water. If the kidneys can keep up with the water coming in, then frequent bathroom trips for urination would occur when a person drinks a lot of water.
However, if the kidneys can't keep up, that's bad. The question then should be "what happens when you drink too much water?" An intake of too much water leads to a condition called hyponatremia (water intoxication). It can lead to death. What happens is that the water creates imbalances of sodium levels in the body. Sodium is used to balance all kinds of fluids in the body, so that imbalance can cause fluids to move from your blood into your cells. When the cells begin taking on too much water, they swell. The swelling is happening all over, so water intoxication causes cells inside the brain to swell.
A few signs and symptoms of hyponatremia are nausea, vomiting, headaches, confusion, seizures, coma, and death.
When you drink a lot of water, you add water to your system (blood stream, organs etc.) Your cells will soon become hypotonic, or swell, which can be a serious health issue for you.