Evolutionists have a bank full of stories used to support evolution which sound good on the surface. I think I first heard this one in a junior high school assembly during a "Mr. Wizard" film. The wise and believable professor in the film convinced his young assistant that evolution had to be true by showing how similar human blood is to seawater. The fact that the same chemicals (such as salt) are in each "proves" man's relation to all of life, which originated in the sea. But as with many of the evolutionary tales, this one doesn't stand up to scrutiny.
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. To be fair, this argument for evolution seldom appears in modern textbooks. Its prominence in textbooks from 1940-1970, however, insures its continuance by those who learned it then. It's indelibly imprinted in evolutionary folklore.
The second point which could be made is that the claim for similarity is made comparing human blood to modern-day seawater, but shouldn't it be to seawater long ago, when life arose and the precursors of blood first evolved? We know that the ocean's salinity increases each day, as rivers dump their dissolved solids into it. Evolutionists propose (without evidence) a steady state for the ocean, but wouldn't the concentrations have changed throughout its over (supposedly) three-billion-year history? Maybe the differing concentrations between blood and seawater better prove evolution. Maybe that's their next claim.
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.
No, blood and seawater are quite different, but even similarity wouldn't prove evolution. Nothing could prove evolution, 'cause it isn't true.