Anyone who has played in the ocean knows getting a little sea water in your mouth and swallowing it will not kill you.
Seawater contains salt. When humans drink seawater, their cells are thus taking in water and salt. While humans can safely ingest small amounts of salt, the salt content in seawater is much higher than what can be processed by the human body. Additionally, when we consume salt as part of our daily diets, we also drink liquids, which help to dilute the salt and keep it at a healthy level. Living cells do depend on sodium chloride (salt) to maintain the body’s chemical balances and reactions; however, too much sodium can be deadly.
Human kidneys can only make urine that is less salty than salt water. Therefore, to get rid of all the excess salt taken in by drinking seawater, you have to urinate more water than you drank. Eventually, you die of dehydration even as you become thirstier.
If I am thirsty and I drink sea water, it will make things worse. During normal activities if you have taken in some sea water, you will naturally consume enough fresh water (or fluids) to flush excessive salt out.
Maybe the real question is; if you drink sea water, how much fresh water do you need to drink to off set the sea water you drank? For every 1 unit of sea water, how many units of fresh water do I need to act as an antidote?
As pointed out in answers to the question How safe is drinking distilled water? it takes a lot of energy to make fresh water from sea water. Assume you are in a survival situation, with only sea water available, one of your companions has consumed a quantity (1 unit) of sea water.