William Chisholm & Sarah Cook mar. 1821 Bristol

My 3 x great grandparents William Chisholm & Sarah Cook married at St.George, Bristol, England on 22nd October 1820.
They had 2 sons both baptized at St.James, Bristol, these were William Crane on 12th August 1821 & my 2 x great grandfather Henry (James) Chisholm on 14th September 1823.
I have been unable to find the elder William Chisholm's baptism in the Bristol area, nor William & Sarah Chisholm in any census nor their deaths?
On the 1820 marriage entry William Chisholm was listed as a sailor so he could have come from anywhere.
If William & Sarah died in the Bristol area it is likely to have been before registration began in 1837.
As they had 2 sons baptized in quick succession then no other children it could mean that either Sarah died or the couple moved to another part of Britain or even emigrated.
Any leads would be much appreciated?