That's an amazing explanation and thoroughly comprehensive answer.

Originally posted by knighttemplar1960:

The only thing that I can think of that requires a salt water biome to survive in game is a barnacle. Perhaps only the deep waters in Valheim are salty or the fictitious abyssal barnacle is an unknown species of fresh water barnacle.

I don't know about barnacles, but that's close to how the Baltic works. It's saltier deeper down since saltwater is more dense than fresh water.