I also deem it self defense. You had no way of knowing all that that person would do to you...what makes you think he would not have killed you?
Please don't continue to put this off... PTSD does not get better with time... begin working on this now, I agree with your Scout leader... it can torment your very soul and ruin the rest of your life. You don't have to come clean as though it was a murder, though, imo. But you do need to go through the event with someone who knows how to help you.
You might not even be remembering it all correctly... the brain does that with trauma... you may have been more threatened than you are recalling.
Yes, to make someone let go of you in the water, if they are panicking, is to go under... but did you know that then? It sounds like since you know how to do this now... that maybe that is all you should have done then... and of course, how could you of? Often when a larger person grabs someone in the water, they both drown. You rescued yourself. I'm sorry you are still suffering because of an attack upon YOU.
Please go to the T... and tell the story with how you aren't sure you are remembering all the details... and you are so suffering for this.... You were a child... and did a very good job of keeping yourself safe!