Why warships avoid raw seawater engine cooling

Why not cool warship engines directly with raw seawater?

Saltwater corrodes and fouls machinery, so naval systems normally keep treated coolant separate and use seawater only through a heat exchanger.

River and lake boats can sometimes tolerate raw-water arrangements because freshwater is less corrosive and their systems are simpler. Ocean-going warships instead isolate treated coolant from seawater through heat exchangers, limiting salt deposits, corrosion, and long-term machinery damage.