Nathan Hibbert struck Callum Porter in an incident near a bar in Helensburgh in December 2010.
Mr Porter, 25, was badly hurt and only recently came out of rehabilitation.
Hibbert went on trial at the High Court in Glasgow accused of attacking Mr Porter. The 23-year-old admitted hitting Mr Porter but said he did it to stop him assaulting a marine.
Hibbert – based at Faslane in Helensburgh – was acquitted after a jury returned a not guilty verdict.
The trial heard how Mr Porter required an operation on his skull and has been put on restricted duties in the Royal Navy.