You need to call me to discuss aluminum heads in salt water. There is a BIG issue with bolting heads based on gen IV heads with round water passages on gen VI blocks with oblong water passages. Too long to relate to you on here but we can discuss. I have personal experience with this, you are right to be skeptical. Some people get away with aluminum heads in salt water but I did not and I will tell you what to look for.
I have posted pics before, do a search.