The Bronze Star didn't exist at the time of the loss of Wahoo (11th October 1943; the award came into existence in 1944). It may easily have been awarded retroactively and posthumously for valor (connected to the Purple Heart, which was a regular pairing, and citations back to 1941 were eligible for it) but if you want to know what for specifically, you may have to apply to the National Personnel Records Center in St. Louis with a FOIA request on Standard Form 180.