Harry, you have a 1979 boat with a 1979 engine operating manual?
Put it in perspective. In 1979 oils did not perform the way they do today, oil additives broke down way faster than they do today. If your manual calls for 20W30, think about what that means. Your engine needs 30 for normal operation, and Volvo is saying that's too thick for cold starting.
Today's 0W40 and 5W30 oils DID NOT EXIST IN 1979! But they would be fine for your engine (provided they are C-rated, not just S-) today. Volvo probbaly would have spec'd a 5W30, IF IT HAD EXISTED IN A QUALITY OIL PRODUCT BACK IN 1979.
If you do a web search on what the SAE and API numbers mean, the mystery goes away. Does a redhead or a blonde make a better spouse? (SAE numbers and hair color are both about equally relevant today.)