Seawater is the water in the sea: it is salty, and you cannot drink it. It is simply sea + water, and English has had the compound since Old English (sǣwæter). People also say salt water, but that phrase is wider — a salt lake is salt water too, and so is water you have added salt to. Seawater always means it came from the sea. You will hear it in science class, in news about the ocean, and in everyday talk at the beach. Nearby languages built the same idea the same way (Dutch zeewater, for example), which is what you would expect for such a plain pairing.

海水