This is a short but unforgettable day, and the key is leaving early.
Take the first morning ferry from Porto Antico Genoa to Camogli — arrive before the day-trippers and walk the village in peace. Grab breakfast at a bar on the main street, then board the small ferry from Camogli harbor directly to Abbazia di San Fruttuoso.
Spend the morning here. Explore the abbey, snorkel above the Cristo degli Abissi if the water is calm, eat lunch at the single small restaurant on the beach. This is one of those places where missing your planned ferry back is not a disaster — it's an upgrade.
Take the afternoon ferry back to Camogli, grab a late aperitivo at one of the waterfront bars and head back to Genoa.