Entertained us, plunging into a sea (6)
I believe the answer is:
amused
'entertained' is the definition.
(amusing is a kind of entertaining)
'us plunging into a sea' is the wordplay.
'plunging into' indicates putting letters inside (I've seen this in other clues).
'sea' becomes 'med' (short for Mediterranean).
'a'+'med'='amed'
'us' placed inside 'amed' is 'AMUSED'.
(Other definitions for amused that I've seen before include "was entertaining" , "Tickled pink" , "Beguiled" , "Medusa (anag)" , "Kept someone entertained" .)