Diddy, you're brilliant! And John Wigham-- your John Humble WAS Captain John Humble's grandson.
Diddy's breakthrough hints were spot on: I'd been on the wrong track, looking in South Shields because I'd read that Captain Humble was "of Shields." As it turns out, Captain Humble lived in North Shields.
I'm still researching, but here's what I've found so far: As Diddy suggested, the Bishops Transcripts for Durham, which include Northumberland, yielded clues as to Captain Humble's family connections. From these records, I learned that Captain John Humble and his wife Ann Alexander-- who lived in Toll Square, North Shields but were natives of nearby Newburn--had at least two sons and five daughters.
Here's the record with the most info I've found so far: Baptized 8 December 1797: "Margaret Humble, 5th Daughter of John Humble of Toll Sq., Master Mariner, Native of Newburn by his wife Ann Alexander, Native of Newburn." [From Bishops Transcripts of Durham: Northumberland, Tynemouth, 1762-1813. Image # 256 of 835."
Image # 192 of 835 in this same set of records shows that William Brown Humble--the father of John Wigham's ancestor John Humble--was indeed the son of this John Humble who was a master mariner.
Did a quick search for Newburn marriage records-- lots of Humbles in Newburn, but seemingly not Captain John Humble and Ann Alexander. You'd think if they were both from there that they would have married there. Lots more to research.
Thanks so much Diddy-- for all of your great suggestions!
Jenny