Well... I'm sure that for some period of time, earlier in his career, like many sailors, he was simply referred to as "seaman".....
I think he would have been referred to as a "seaman" on every ship where he wasn't the Master.
I've just dug out my "Seaman's Discharge Book" (no kidding it really is called that) from my brief period as a ship's engineer and in the Certificates of Discharge section - essentially a logbook of engagements and discharges from various ships- there is a column for the Signature of the Master so Captain Bates of the QM2 would have signed these as Master. Tucked into its pages I also found a Testimonial of Service and that too bears the signature of the Chief Engineer and the Master and the book includes many references to the Master (and none at all to Captains) .
I seem to remember that many traditional Masters of cargo ships were rather sniffy about being referred to as "Captain" a title they associated with all the Captain's Table and ballroom dancing with the wealthier female passengers nonsense that in their view had nothing at all to do with the master mariner's real job of making a successful passage.
The snootier shipping lines were at one time quite fond of awarding their senior captains the equally legal status free title of Commodore.