Install the Blue Sea 8100
https://www.bluesea.com/products/8100/ELCI_Main_30A_Double_Pole
into a Carlon indoor/outdoor J-box near the shore inlet. This will run you well under $200.00... Regardless of what the standards say, your insurer really has the last word. They are all getting skittish on ESD....
If you have a standard toggle breaker AC panel then the Blue Sea A-Series toggle ELCI only uses one additional space in the panel. Still dropping one into a Carlon box in the lazarette is often easier....