There are two ways to ingest seawater -- at the top and at the bottom. The body's response will differ depending on the route
of entry, but the end result will be the same -- further dehydration.
If you drink sea water, much of it is absorbed and increases the salt concentration in your blood stream and fluid outside cells. The salt increases the osmotic load in those fluids. Water will leave your body's cells to equilibrate with this increased osmotic load. The kidneys try to dump the excess salt. Unfortunately, while their capacity to concentrate salt into the urine is impressive, they can't achieve the salt concentration of sea water. As a result, more water than salt is excreted leading to worsening dehydration.
Not all the salt water is absorbed so it will move downstream to where it meets the water put in the, um, other way. In the colon, the body usually tries to absorb water before leaving semi-solids behind. Unfortunately, it uses osmosis
to help achieve this and the osmotic effect is a two way street. If there is a higher concentration of salt in the colon than the blood and extracellular fluid, water flows from body to colon to out causing osmotic diarrhea and further dehydration.
Unfortunately, at ocean salt concentrations, the "Water, water everywhere nor any drop to drink." adage is true.