Internet is full of misleading information. There is no scientific research to prove this contention. Coconut water was used as plasma substitute in WWII.
But this contention is dangerous as seawater can kill if used as plasma.
This contention is similar to the philosophical question:
According to John D. Morris, Ph.D.:
"First off, while both contain many of the same salts, concentration of dissolved particles in blood is very different from that in seawater. The primary constituents of both are sodium and chlorine (which together make up common salt, NaCl), but seawater has three times as much sodium and five times as much chlorine per unit weight. Hardly the same. Furthermore, it contains eight times as much calcium and fifty times as much magnesium.
On the other hand, some dissolved salts are greater in blood than in seawater. Blood has two hundred times more zinc, two hundred and fifty times more iron, and one thousand times more copper. All of these (and many others) found in both are common minerals found everywhere, in both organic systems and inorganic rocks. Of course they would be found in blood and seawater as well as inorganic clay. Since the suite of these minerals is so different in blood and seawater, it seems unwarranted to claim that one comes from the other.
Finally, as far as blood is concerned, it's not just the mineral concentration that's important. The cell's DNA is designed to use these minerals to form extraordinarily complex molecules, building structural proteins and enzymes, etc., utilizing the mineral ions to transport nutrients, conduct electrical impulses, barricade out harmful chemicals, and a host of other functions. This is true for both single-celled organisms and multi-celled plants and animals. All of this is far different from anything in seawater. In fact, seawater destroys these necessary constituents of life."