First of all, I'm sorry to hear what happened and your family has my condolences.
I remember being taught if wearing a shirt to float on your back and splash air under it to make a balloon and just try to get yourself somewhere safe. If no shirt, do the best you can to get somewhere safe, even if it isn't where you came from.
I can relate to the unmanned boat problem. In April, I was fishing along in about 20 of water with the trolling motor on cruise control set to slow speed. I went to put a rod down on the deck to switch to different one and mis-stepped to the floor at the same time a boat wake caused my boat to rock. It caused me to lose my balance and I went over the starboard side in a flash.
I lost the rod in the water because I hadn't quite let go of it and it went overboard with me. It was cool at 6am, so I was wearing a long sleeve cotton shirt, jeans and hiking boots. Water temps were in the low 60's. It seemed like forever to get to the surface and I had to work to stay afloat. Luckily, I had the remote control around my neck for the trolling motor and was able to stop the boat before it got too far away. I was freezing. If I hadn't had the remote around my neck, I wouldn't be typing this because the boat would have just kept going away faster than I could swim.
I always wear a life jacket and kill switch when I'm driving the boat because all it takes is a slight hiccup at 60 mph to throw you out of the boat. However, I never wore a life jacket while fishing. Now I wear an Onyx A-24 while I'm fishing for protection against going overboard....