Um, no idea what those intakes really look like....but, if it is cast iron, then the casting must be fairly thick and complete rust-through seems highly unlikely, no? I'd say dunk it in that cleaner, but probably will need mechanical means to remove heavy rust. Don't those marine boat engines use the surrounding water to cool the engine? How does that work if boat is in the sea with salt water? Can you post an overall photo? I'm not sure how you would get in there though. Maybe a die grinder with a flap cookie disc or scotchbrite disc?