Sceptic coasting at sea (8)
I believe the answer is:
agnostic
'sceptic' is the definition.
(I've seen this before)
'coasting at sea' is the wordplay.
'at sea' is an anagram indicator.
'coasting' with letters rearranged gives 'AGNOSTIC'.
(Other definitions for agnostic that I've seen before include "He finds it difficult to believe" , "One believing God is unknowable" , "Coasting (anag)" , "Doubter of God" , "Person who doubts the existence of God" .)