Comment about mariner's first boat (6)
I believe the answer is:
remark
'comment' is the definition.
(a remark is a spoken comment)
'about mariner's first boat' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'mariner's first' becomes 'm' (1st letter of 'mariner').
'boat' becomes 'ark' (ark is a kind of boat).
're'+'m'+'ark'='REMARK'
(Other definitions for remark that I've seen before include "Make mention of" , "Incidental observation" , "Mention - observe" , "(E.g. Virgil's) pastoral poems" , "Something said; take notice of" .)