Material for exhaust manifolds of marine engine

I am developing water cooled exhaust manifold for marine engine. The ex. manifold is cooled by sea water during operating conditioning.

Aluminium can be a better option for the manifold.Therefore please suggest which aluminium alloy should I use for this application?

Most exhaust manifolds made from cast iron although there are some companies that make them out of stainless(expensive). If you use aluminum and the block is cast iron, will have galvanic corrosion with seawater going thru. If the engine is gong to be fresh water cooled, oversize heat exchanger and use coolant to cool exhaust manifold. It will last a lot longer that way. My diesels on my boat have fresh water cooled exhaust manifolds and they have lasted 15 years with only the dumps having to be replaced.