« Reply #18 on: Wednesday 23 May 12 21:08 BST (UK) »
not exactly an unusual occupation in the area, plenty of mariners around.
There are no birth records prior to civil registration, only baptisms. Most parish baptisms from the period exist, but as one or other of them could them be non-conformist, it's unlikely that a parish baptism would exist, parish baptisms being Church of England at the time. Cardigan St Mary baptisms are online, but no sign of one for John or Jane, & none for their children.
Have you had a look for the fathers on the Welsh mariners site? There are entries for each name that could fit.
Cornwall: Allen, Bevan, Bosisto, Carnpezzack, Donithorn, Huddy, James, Retallack, Russell, Vincent, Yeoman
Cards: Thomas (Llanbadarn Fawr)
Glam: Bowler, Cram, Galloway, James, Thomas, Watkins
Lincs: Coupland, Cram
Mon: Cram, Gwyn, John, Philpot, Smart, Watkins
Pembs: Edwards (St. Dogmael's)
Yorks: Airey, Bowler, Elliott, Hare, Hewitt, Kellett, Kemp, Stephenson, Tebb