Lithium content varies as much as salt content, but one thing is certain: the open seas are consistently salty.

That's only if you take the average of all the measurements collected across the entire ocean for decades. In one spot, salinity is not consistent. Patterns of evaporation and rainfall affect not only salinity but also the presence of organic matter and other random contaminants. Such fluctuations are most pronounced at the surface, where this device is meant to operate.

It's the substance that composes those 75% of the surface of our planet that we call "sea".