In the attempt to prevent or treat dehydration you should not drink sea water.
In short: By drinking seawater you ingest excessive amount of sodium chloride, which needs to be excreted by urine in order to maintain normal blood sodium concentration. Since the human kidneys have a limited ability to concentrate urine, the amount of water lost by urine in order to excrete sodium chloride is greater than the amount of water in the drunk seawater. This results in a net water loss.
Detailed explanation is below:
E. Hall, Guyton and Hall Textbook of Medical Physiology, 13th edition, 2016 (p. 373)
The limited ability of the human kidney to concentrate the urine to
only 1,200 mOsm/L explains why severe dehydration occurs if one
attempts to drink seawater.
Sodium chloride concentration in the oceans averages about 3-3.5% with
an osmolality between about 1,000 and 1,200 mOsm/L.
Drinking 1 liter of seawater...would provide a total sodium chloride
intake of 1,200 mOsm. If maximal urine concentration ability is 1,200
mOsm/L, the amount of urine needed to excrete 1,200 mOsm would be...1
The kidneys must also excrete other solutes, especially urea, which
contribute about 600 mOsm/L when urine is maximally concentrated.
Therefore the maximal concentration of sodium chloride that can be
excreted by the kidneys is about 600 mOsm/L. Thus for every liter of
seawater drunk, 1.5 liters of urine volume would be required to rid
the body of 1,200 mOsm of sodium chloride ingested in addition to 600
mOsm of other solutes...This would result in a net fluid loss of 0.5
liter for every liter of seawater drunk.