Hi Moya
As you're no doubt already aware, the 1851 Census gives Liverpool-born pilot William DANIELS' age as 58, placing his birth year as 1792/1793.
In the 1841 Census Lancashire-born pilot John DANIELS' age is given as 40, placing his year of birth (given the rounding-down to the nearest 5 years) as anytime between 1796 and 1801.
The IGI has baptisms in Liverpool St Nicholas as follows - all children of John DANIEL and Ellen :-
1788 Elizabeth
1790 Sarah
1792 William
1795 Henry (mother Ellen GARRETT)
1800 John (mother Ellen GARRAD) (born 19 November 1799)
So the prospect of Liverpool pilots William & John DANIELS being brothers looks pretty good, the one nagging doubt being this baptism, also on the IGI :-
14 September 1800
Liverpool St Peter
John DANIELS
Father John DANIELS, mother Jane
Regards
Gordon