Since you are coating aluminum castings for salt water marine water environment you will need excellent corrosion protection. Therefore, I would recommend zinc chromate over zinc phosphate. Many companies are moving away from zinc chromate because hexavanlent chromium is known to be a carcinogen. Therefore, if you decide to go that route, please be aware of OSHA's stringent regulations controlling hexavalent chromium (Cr VI) exposure.
You are permitted to use the primer, but you are obligated to control the amount of Cr VI in the air and you must provide your painters with appropriate personal protective equipment (PPE).
I have not seen articles showing that zinc phosphate is equivalent to zinc chromate in terms of corrosion protection.
Finally, please be aware that the most critical aspect of the coating process is surface preparation of the aluminum castings. That is more important than your choice of zinc chromate versus zinc phosphate.