Inevitably there will be times when you 'know' your ancestor was a British merchant seaman. See the posting on tracing 19C merchant seamen. It is easier than you may expect, to miss a section.
Many registers give places of birth.
Look closer at those claiming to be born in the same county as your man. Don't discount someone purely on this. He's still not there. Try all the variant spellings of his surname. Some men did slip through the net and managed to work without ever getting a ticket.