Plunging (6)
I believe the answer is:
diving
'plunging' is the definition.
('dive' can be a synonym of 'plunge')
This is all the clue.
(Other definitions for diving that I've seen before include "Plunging oneself into water" , "Throwing oneself into water" , "Water sport" , "Plunging into water - falling in the air" , "Going under water" .)