Most likely culprit is something wired through the boat’s harness. Engine is rarely the source of issues with parasitic drain. Might be worthwhile to also remove and clean each connection on the terminal block in the battery compartment as often in a rig of that age it can be the source of electrical issues.
BCB usually mounts that breaker directly to the positive post of the battery with a short metal strip. Sorry I don’t have a pic because I removed mine and added a fuse block. Have you verified that it is parasitic drain by charging the battery, removing all connections to isolate the battery, and then monitoring charge over a few days?