First of mariners in a boat at sea somewhere in Canada (8)
I believe the answer is:
manitoba
'somewhere in canada' is the definition.
(Manitoba is a Canadian province)
'first of mariners in a boat at sea' is the wordplay.
'first of mariners' becomes 'm' (1st letter of 'mariners').
'at sea' indicates anagramming the letters.
'in'+'a'+'boat'='inaboat'
'inaboat' anagrammed gives 'anitoba'.
'm'+'anitoba'='MANITOBA'
(Other definitions for manitoba that I've seen before include "Strange boatman I find in Canada" , "Is Oman a bit in Canada, strangely?" , "northern territory" , "sparsely-populated province" , "Canadian province, capital Winnipeg" .)