The limited ability of the human kidney to concentrate the urine to a maximal concentration of 1200 mOsm/L explains why severe dehydration occurs if one attempts to drink seawater. Sodium chloride concentration in the oceans averages about 3.0 to 3.5 %, with an osmolarity between about 1000 and 1200 mOsm/L. Drinking 1 liter of seawater with a concentration of 1200 mOsm/L would provide a total sodium chloride intake of 1200 milliosmoles. If maximal urine concentrating ability is 1200 mOsm/L, the amount of urine volume needed to excrete 1200 milliosmoles would be 1200 milliosmoles divided by 1200 mOsm/L,
or 1.0 liter.
Why then does drinking seawater cause dehydration?
The answer is that the kidney must also excrete other solutes, especially urea, which contribute about 600 mOsm/L when the urine is maximally concentrated. Therefore, the maximum concentration of sodium chloride that can be excreted by the kidneys is about 600 mOsm/L. Thus, for every liter of seawater drunk, 2 liters of urine volume would be required to rid the body of 1200 milliosmoles of sodium chloride ingested in addition to other solutes such as urea. This would result in a net fluid loss of 1 liter for every liter of seawater drunk, explaining the rapid dehydration that occurs in shipwreck victims who drink seawater.
Source: Textbook of Medical Physiology - Guyton and Hall