You have a few questions to answer.
Yes it is raw water cooled and the engine coolant does not touch the raw water side of cooling system. The raw water flows through more than one item. The raw water runs into the reverse gear cooler, then cools the air passing through the after cooler enters the heat exchanger and then exits through the exhaust manifold.
A engine operated exclusively in salt water that is not flushed may compromise the after cooler tubes. If a tube over time corrodes and starts to leak, injection of salt water could enter the air induction system and cause extensive damage to the engine. (worst case) The raw water tubes get blocked from the build up of sea critters, and corrosion. This reduces after coolers ability to cool the air to efficient levels and either put the engine into derate or you find reduction in turbo boost and performance.
Reverse gear cooler ,this could effect the engine if it is plugged or corroded. It will reduce water flow to the complete raw water cooling causing an over heat. Over heat an aluminum block, aluminum head diesel engine and you may reduce the engines longevity.
Heat exchanger. This is the heatsink of the closed cooling system . It needs to flow raw water though it without reduced flow in order to maintain the engines cooling system. If it becomes obstructed the engine cooling system is less efficient causing a rise in operating temperature. Heat can be a disruptive enemy to a internal combustion engine.
Exhaust riser leak through can cause exhaust cooling sea water to enter the exhaust manifold. If seawater enters the dry side of the exhaust this can damage the Turbo and possibly get into the engine cylinders which could cause a hydrolock, cylinder corrosion, and damage to internal engine parts.
Each one of theses components noted are part of the raw water system , Aftercooler, reverse gear cooler, exhaust riser, heat exchanger. Price them out ! Theses parts will equal the price of a block. Not cheap! So if you engine block makes it through a poorly maintained raw water system you may still be forking out coin for the bolt on components.
Don't worry about the engine failing and getting another engine. Just maintain the one you have. Believe me if you maintain it. The engine will not be a "Throwing it Away". If you are using it on Lake Michigan it is getting flushed every time you use it with clean fresh water. If you take it to salt water areas for a month or two and then use it on Lake Michigan it is flushed again. ( I boat on Lake Michigan Wisconsin side. I don't think I ever jumped in 80 F water. We were lucky to see 70F. I did have a cooler operating engine on the lake 189F to 193 @ WOT 4130. 184F at 3200 rpm. The D3 150 should not see over 178F WOT if properly plumbed. The D3 220 should not see over 185 WOT if properly plumbed.
Enjoy your R25SC D3 150 hp, maintain it and you will be using it for a long time! No need to look for an alternative power plant.