Bachelor requesting quiet on vessel, seeing creature of the deep (7,5)
I believe the answer is:
basking shark
'creature of the deep' is the definition.
(type of shark)
'bachelor requesting quiet on vessel' is the wordplay.
'bachelor' becomes 'B' (abbreviation e.g. BA=Bachelor of Arts).
'requesting' becomes 'asking' ('ask' can be a synonym of 'request').
'quiet on vessel' becomes 'shark' (I can't justify this - if you can you should believe this answer much more).
'b'+'asking'+'shark'='BASKING SHARK'
'seeing' is the link.
(Other definitions for basking shark that I've seen before include "Large fish that often floats on the surface of the sea" , "Plankton-eater" , "Swimmer afloat" , "marine creature" , "Large unthreatening fish" .)