The French fools — they're cut off and wet (7)
I believe the answer is:
lagoons
'they're cut off and wet' is the definition.
(isolated bodies of water)
'the french fools' is the wordplay.
'the french' becomes 'la' ('the' in French).
'fools' becomes 'goons' (goon can mean a foolish person).
'la'+'goons'='LAGOONS'
(Other definitions for lagoons that I've seen before include "Coastal lakes" , "Bodies of water cut off from larger bodies by reefs" , "Shallow waters separated from the sea by sand" .)