if was in Army then 99% certain lived in Castletown with bapt either in Castletown St Mary's (the usual garrison church) or possibly Malew
However you stated Merchant Seaman - Douglas was the main port - for owner skippers it was not unknown for a wife to be on board tho I doubt the Irish sea would be pleasant in late stages of pregnancy.

Have you found her in 1851 or 1861 census ?