First of all, have you checked that 304 or 316 SS are suitable for your marine environment? Marine environment means salty and moist atmosphere, and, as you know, austenitic stainless steels are subjected to stress corrosion cracking in the presence of chlorides. Stress corrosion cracking leads to sudden collapse of the stailess steel after some time of use. I recommend you to check the suitability of the material before going any further.
Giovanni S. Crisi
Sao Paulo - Brazil

You would be better off spending the extra money and using 904L S/S

If the marine enviroment is fresh water, 316 is the best choice because even some fresh water will cause localized corrosion on 304. If it is salt water exposure 904L would be fine, but AL6XN is probably easier to acquire. Make sure you remove any iron contamination from the fabrication process with a pickling solution or citric acid chelant.
Filler metals and welding procedures will depend some on the application and service conditions. Need this information to recommend welding procedures.