Thanks for the replies. I suppose I could just ground to the outboard and keep it lowered in in the water when connected to shore power. I only connect to shore power if staying on the boat overnight. Otherwise I disconnect it when leaving the boat.
You'd better keep a close eye on your zincs if you use the outboard then. Outboard engine zincs are tiny, and will get eaten up quick if one of your neighbors is leaking current into the water.
I recommend using a ground plate instead of your O/B.
To the OP who is using an O/B, but still has the dead inboard engine in place:
Continue using the inboard engine as your ground point. The engine doesn't run, but it's still a large hunk of metal in contact with the water, so it makes and adequate ground point for the bonding between AC and DC systems.