It is an often-repeated claim that human, and in fact all animal blood is salty because we evolved from aquatic organisms, and that blood has a similar concentration of salts as ocean water, or at least as primordial ocean water. 

Is any of that true? Does science know what the salinity of the oceans was billions of years ago, and was it indeed similar to the blood of today's organisms? Does that knowledge prove anything about evolution and the origin of life?

I am wondering, because I tried to Google for the answers, and unfortunately all I could find were creationists referencing each other how it couldn't possibly be true because evolution is a lie in the first place etc., and they claim that there is no empirical research that would compare salinity levels of organims' cells and ocean water.

Short answer
Early sea water had a very different osmolality than blood plasma.

Background
The reference range of serum osmolality is 275–295 mosm/kg (mmol/kg) (MedScape). The osmolarity of sea water is about 1000 mOsm/l (Wikipedia), but it can vary substantially between different seas, namely between 642 and 1,480 mOsm/kg (Ninawe & Banik, 1998). Nonetheless, the range of osmolalities are substantially higher than that of serum. That's why we cannot consume sea water, without losing more water than gaining it (NOAA). According to Astro Biology Magazine, the salinity of early oceans 3.5 billion years ago, when life first developed, was even higher. So popular accounts that they are the same and that this has important evolutionary consequences are likely far-fetched.   

Reference
- Ninawe & Banik, J. Mar. Biol. Ass. India (1998); 53(2): 230-36
