When I was about 2 my dad was side swiped and I went out the open car window. (Before car seats were mandatory). I suffered a bit of road rash and nothing else and ended up with only 1/2 inch scar. 2 year olds are light and floppy so I could believe he could have been launched and scared that night, especially with that angle.
I've also seen a grown man fall out of one of those flat bottom boats. I was walking over a bridge and the boat was traveling underneath. A gentleman stood up to adjust his line and over he went. Luckily someone else was driving and stopped the boat. They were also wearing lifejackets. We stopped and made sure everyone was okay and continued on our way. (Not sure how he got back into the boat). If he didn't have lifejacket on and went over he may not have come up, especially with a medical emergency.
Growing up in MN, catching an hour or even less of fishing was normal for me, especially if I brought my siblings along. Fresh air and excitement entertained them and wore them out faster.
I'm inclined to believe it was an accident and he's in the water. I've read about gators in Lewisville lake, big ones. He could be tucked away for the moment (that sounds harsh, sorry).