their are tons of fresh water boats with aluminum heads.also many closed cooling boats with aluminum heads,merc 525 being one example.

Aluminum heads can be anodized for marine application.

Ran 1992 AFR heads till sold engine in 2014. Freshwater cooled. Water passages had some corrosion but lots of life left.

Saltwater use is another story........

It depends on the lake water - mineral deposits - ph balance - brackish water and so on. In my area we have one lake that just causes havoc with t stat housings - T stats - water circ pumps and when you drain the blocks heavy rusted color water drains out.

Also it corrodes the box frames on the trailers pretty bad as well. Fresh water hose flushing can help for open / raw water cooling engines. We have a lot of mineral deposits as well that causes cooling issues. Especially the water passage in the transom assembly bell housing

Just example of the of the many --- A 2009 Volvo penta 496 MPI one owner and I have worked on the boat since Brand new from day one.. LOCAL LAKES only usage. Engine ran too cold and was in guardian mode because the coolant temp never reached higher than 135 degrees and it ran rich. I had to change the spark plugs as well The boat / engine only had 168 hours. NON closed cooled Volvo had the 496 s both ways - closed cooled apps and NON. Pic below. It had a stuck open T stat from all the corrosion.

i think my motors have had bow tie heads 14044861, since early 90's ..is fresh water boat, last year i made it a closed coolant system.