I looked at the census that you noted with David (cen 1900) being shown as born at sea. I noticed that Mary died in 1902 so had no futher reports. Also all her previous census list "SC". Since this is the only census I could find noting born "at sea" I am wondering if this wasnt a census recording error. You will note that the "at sea" recording could have possibly have be the pronunciation of the abreviation of south carolina "SC". Thanks for the heads up on this. I am currently working on David in SC. I have started a McCombs YDNA project on ancestry.com and am trying to collect dna samples of those McComb(s) families located there. Since those McCombs researchers that I know have no info on David beyond himself, my attempt is to try matching some lines still located in that area.
Bill McCombs (firstname.lastname@example.org