Thank you, that is a useful introduction. After reading it I looked up "heat exchangers" on the net and found this quite thorough article: https://www.marineinsight.com/guidelines/general-overview-of-central-cooling-system-on-ships/
When I was at sea there was a physical distance between deck and engine crew. The engine people lived and stayed on the port side and kept to themselves. As we who worked on deck did on the starboard side. And there was even a low temperature "war" going on, exemplified in that if one from the engine spilled a drop of oil on deck, a delegation was sent down to demand that it was immediately cleaned up by someone from inside the engine casing, even though it would demand nothing more than a short swipe to remove. So naturally it was possible to stay completely ignorant, as I did, of the propulsion of diesel engined ships.