Does animal blood, esp. human, really have similar salinity as ocean water, and does that prove anything about evolution?
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 &amp; 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 &amp; Banik, J. Mar. Biol. Ass. India (1998); 53(2): 230-36