As for the salty sea, as a first approximation, think of all the rocks that are present in rivers that feed the sea. Rocks are made of minerals. Some of these minerals are salts.

Yet, the river is not salty, you might think. That's true, because the amount of salt being eroded is limited. However, these rivers all end up into the sea. And thus, the salt accumulates there. When it's hot, the water from the sea can evaporate and feed the rivers again. However, the salt remains, and over time, more and more salt ended up in the sea, thus making them more salty than the rivers.

I hope this helps :)