Film that justified all water safety tips! (4)
I believe the answer is:
jaws
'film' is the definition.
(I've seen this before)
'justified all water safety tips' is the wordplay.
'tips' indicates taking the first letters (tip can refer to the top end of something).
The first letters of 'justified all water safety' is 'JAWS'.
'that' acts as a link.
(Other definitions for jaws that I've seen before include "Bond villian" , "lectures" , "Talks at length; grippers" , "Gripping part of wrench" , "rabbits" .)