How To Weld Stainless Steel To Mild Steel. 316 would be better,especially if you wish to leave the weld as a feature. 98% argon and 2% oxygen.
A dissimilar rod or 309 will corrode with the salt water, but so will mild steel. And this means you have to have a clean joint.
1KG Price For 25mm Or 32mm Or 40mm Stainless Steel A304
And you can also weld well with a 98% argon and 2% carbon dioxide mix. And your flux core welder must be capable of outputting at least 90 amp.