Stainless and lead with a zinc connected to all of them?
Zinc is more reactive then iron, and iron more than lead. I think the zinc would help both.
I understand that lead is quite stable in salt water, and more so in fresh water.
Be wary at the interface between the stainless and the lead. It is likely to get oxygen depleted. Do your best to keep chlorides (borne by salt water) out of there. It will not be easy perhaps, but stainless is very unforgiving in oxygen-depleted, salty conditions. It really must be oxygenated in chloride-rich conditions, or is reacts quickly with the chlorine.
I would be more trusting of carbon steel keel bolts in those conditions. It is a more predictable material when oxygen gets depleted.