In the scene I remember, Thor repeatedly dunks Loki when they're going swimming together as children. Loki can't get him to stop, so he deliberately inhales water and starts drowning for real, which causes Thor and their parents to freak out. Thor gets punished and leaves Loki alone after that, and I think the lesson Loki takes from the event is that you can turn weakness into strength if you don't mind getting hurt in the process.
Does this sound familiar to anyone? Any help with identifying the fic would be greatly appreciated! :-)