I believe the head is aluminum but the block is iron. I could be wrong though. At the time these were the lightest diesel engines on the market.
The engine diagrams for the AC-1Wm show a zinc anode but I have never found mine. I was planning to pull a couple of the block's drain plugs and fit pencil zincs but I have yet to do so. My boat is in fresh water so this is a low priority.
As a raw water cooled engine, there is little you can do regarding corrosion when in use. However, 99% of time the engine will not be running (ie. boat sitting at mooring or on long passage) so I think the trick is to flush the salt from the engine. To do this I am in the process of fitting a small water tank for fresh water. Then, before shutting down, I can throw a couple of valves and let the engine's water pump replace all the salt cooling water with fresh.
Another option is to convert to fresh water cooling. There are kits available from a guy who does work for Atomic 4s. Google Atomic 4 and Indigo and you should find him.
How long will you be in the salt? This may influence your decisions.