If life began at sea, why organisms are less salty than the sea?
Many biological processes require regulation of the salts (or more precisely ions) in fluid compartments of an organism. These processes include muscle contraction, filtration of the blood by the kidneys, and so on. Without these processes, a fish would certainly be dead. So it's not necessarily that regulating ions aids evolution or metabolism, nor that the sea changed salinity - the reason is that many biological processes required for an organism's survival wouldn't work without proper regulation of ions.

One such important process is active transport. In essence, by creating gradients of ions across a cell membrane, you can let the flow of ions into the cell drive other processes. This is a bit like how the flow of water through a water wheel can drive other machines. 

I've never come across any reference to a cell with an ion concentration of its intracellular fluid greater than that of the ocean, though I'd slightly doubt that to exist since it would disrupt active transport. I'd also doubt that you could discern the ion concentrations of any fossilized cell since ions need to be dissolved, but how would you see this in a fossil?

I'm not sure whether I've completely answered your question, so let me know if something's still unclear.