Unsound ocean vessel? (5)
I believe the answer is:
canoe
'vessel?' is the definition.
(canoe is a kind of vessel)
'unsound ocean' is the wordplay.
'unsound' indicates anagramming the letters.
'ocean' with letters rearranged gives 'CANOE'.
(Other definitions for canoe that I've seen before include "Primitive boat" , "Narrow paddle boat" , "Light, narrow open boat" , "Small boat for paddling" , "Kayak-like boat" .)