Painter’s picture: a ship on top of ocean (7)
I believe the answer is:
picasso
'painter's' is the definition.
(Picasso is an example)
'picture a ship on top of ocean' is the wordplay.
'picture' becomes 'pic' (short for picture).
'ship' becomes 'SS' (prefix in ship names eg SS Great Britain).
'on top of' says to take the initial letters.
The first letter of 'ocean' is 'o'.
'pic'+'a'+'ss'+'o'='PICASSO'
(Other definitions for picasso that I've seen before include "Pablo, great Spanish painter" , "20th century painter" , "famous artist" , "So aspic is out for the Spanish artist" , "Pablo -, Spanish painter/sculptor" .)