Heroes are born in a split second. I've interviewed some wonderful ones. They all share the same thoughts, that there was NO time to think about their own lives being in jeopardy, but that someone needed immediate help in order to survive. I'm betting that even if that was a stranger in the water, the friend (and now probably best friend) would have jumped in anyway. Personally, I believe that it is a character traite, deep within one's soul. We hear of too many times when others stand by and do nothing. Drives me crazy seeing actual film of such things happening, and instead of the filmer dropping the phone to assist, record the situation. What we call courage, they call part of life, and usually seriously down play their part.
Me, I have no fluffy happy thoughts regarding aggressive big fish with sharp teeth, and think only a whole lotta love regarding the human bait would have me hurling in as a wedge between the shark and his favorite meal.