A barb can, in actuality, be up to six inches long. In this case, he unwittingly frightened the ray, it's tail lashed out, and the barb entered just beneath Steve's ribcage bearing slightly to the left, and fully penetrated/perforated his heart, front to back.
In tandem with the poison (though that is usually not considered fatal) he was dead virtually instantaneously. Extensive efforts were made to revive him via CPR, etcetera, but the location, size, and severity of the wound made those attempts naught but an exercise.
Even had a surgical team been on the boat on standby, sterilised and fully gowned up, with a fully equipped O.R. as it were, he could not have been saved.
He will be missed by all who respected the natural world, and lived a life at it's edge!
Requiescat In Pacem
A.A.C.
Scientific American interview with crocodile hunter Steve Irwin, March 23, 2001.
http://www.sciam.com/article.cfm?articleID=0000462F-9484-1C75-9B81809EC588EF21