Some marine engines do cool on just raw water. Outboards do and some I/O's and inboards. In this system seawater circulates around the block and threw the manifolds. I would not buy a boat that I plan on keeping longer than 5 years thats not fresh water cooled. That is usually how long it takes the salt water to eat threw the manifold and fill the cylinders. However raw water cooling is fine on lakes and rivers, just as long as theres no salt. I am going to ask my dad if the generators on the sub he was on used anti-freeze or were just raw water cooled. It's a newer nuclear boat though, not anything like WW2 subs.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200