“The Merchant Marine” refers to the totality of merchant seafarers, or a specific body of such, e.g., the student body of a Merchant Marine Academy. Individual members of the merchant marine are merchant mariners. Your father may have been an excellent seafarer, but he was never a merchant marine, unless he suffered from a multiple personality disorder.
And I’m not trying to “steal your joy,” but details matter in proportion to the pride you take in your work. I’ve worked with many fine seafarers, but when they call the galley a “kitchen,” it dulls their shine just a smidge.