Finding Nemo doesn't really start the way we think it does I'd like to suggest that Finding Nemo doesn't start the way we are lead to believe, but in fact, what really happened is:

Everyone survived the barracuda attack. While Marlin is passed out, realizing that he moved the family to a terrible neighborhood and that Marlin is a terrible father, the wife takes all the kids and leaves him. Leaving only the disabled child for him to raise on his own. 