Swimming at great event on the water (7)
I believe the answer is:
regatta
'event on the water' is the definition.
'swimming at great' is the wordplay.
'swimming' is an anagram indicator.
'at'+'great'='atgreat'
'atgreat' is an anagram of 'REGATTA'.
(Other definitions for regatta that I've seen before include "Aquatic activity" , "Meeting at Dartmouth, perhaps" , "Boat race-meeting" , "races at sea" , "Boat festival" .)