Despite the accumulation of “curious” substances that the seawater of the beaches, for a drink we will not die. But what would happen if we devote ourselves to drinking salt water (speaking of one with 33 grams of salt per liter, like that of the sea) is that the concentration of salts of our fluids is much lower (9 grams of salt per liter of water), and when trying to match concentrations by osmosis the cells would release too much water shrinking, ending in extreme dehydration when trying to expel the salt with urine (demanding more water than ingested).
The Wonders Of Osmosis
When trying to match concentrations by osmosis the cells would release too much water shrinking, ending in extreme dehydration.
Hence, the fluids of many of the marine animals have the same concentration in salts as seawater. As they reminded us in Naukas, among them we find bivalve mollusks, echinoderms (starfish, hedgehogs), sponges, anemones and many others, all of them capable of having a “you to you” of concentrations and avoiding dehydration.
However, others whose design was not refined so much in this regard although it seems quite the opposite: the internal fluids of marine teleost (and other groups) have a lower salt concentration than that of saltwater.
Thus, even if hake, sea bream, eels, and other species live like fish in the water, they require special maneuvers to avoid dehydration to drink saltwater.
So, What Is The Trick?
Avoid the absorption of salts (of all) to keep the water, expelling them outside. The salts that are not absorbed remain inside the intestine along with the rest of the unwanted food elements. From there they are expelled outside via the gills and urine, but to avoid dehydration they produce very little (and very salty).
In fact, they have quite different kidneys than the rest in terms of structure and functionality. The nephron is the basic functional and structural unit in a kidney. In the kidney of an upper terrestrial vertebrate, such as humans, cows or cats, one can see structures that we sometimes do not see in those of teleost fish. This may include the renal corpuscle (where filtration takes place) or the distal contoured tubule (where additional filtration happens). This has explanation in detail in this study of the University of La Plata (Argentina).
So, Do The Fish Drink Salt Water?
Yes, seafarers have to drink this water. But the thing changes for the river, whose concentration of salts in their body fluids is higher. Drinking would happen to us unlike drinking salt water. Your cells would absorb water to match concentrations with the outside. This shall firstly cause swelling, and everything will eventually explode.