There's no mention of any thrashing about in any of the stories & videos on
http://www.shallowwaterblackoutprevention.org/
This type of death comes silently & quickly. Irreparable brain damage occurs within a couple of minutes.
The people in your video experienced convulsions because they didn't stop breathing/inhaling oxygen.
When you pass out from cerebral hypoxia with your face underwater, you can no longer raise your oxygen levels & regain consciousness. Your lungs fill with water & you don't even feel it because your brain is so deprived of oxygen it can't register pain.
Here's "Chandler's story" from
http://www.shallowwaterblackoutprevention.org/
Myself, my dad and Chandler were swimming in the resort pool, playing the familiar game of seeing how far we could swim from one end of the pool to the other without taking a breath. Mom, Addi (older sister) and Erin (Chandler's wife) were watching from their pool chairs. Chan said he could go from one end to the other without taking a breath. Understand, this is a game we play all the time so nothing seemed out of the ordinary. Chan grabbed scuba goggles and was off underwater. And he did it, going from pool end to pool end without a breath.
After Chan touched the wall, he stayed underwater, sitting at the bottom. My dad and I saw bubbles at the surface and thought that he was just seeing how much longer he could last holding his breath. My mother was visibly worried and upset (thank God for "mother's intuition"); and started saying, "Someone needs to get him up now!" Dad and I insisted he was fine, as we saw bubbles come to the surface of the pool. About 20 seconds passed and finally, Dad went to pull Chandler up and instantly knew something was wrong. He was limp and lifeless and the second my dad lifted him up, we saw his grey skin and blue lips. Chandler's heart had stopped.
My dad immediately started mouth to mouth in the pool as we carried his lifeless body to the stairs. Mom screamed for help and Erin and Addi got up in shock. Once we got to the stairs, there was a man who let us know he was an ICU doctor. He immediately started chest compressions on Chandler, yelling out that he didn't have a pulse. In the midst of all this happening, another doctor came over and assisted. Between the compressions and mouth to mouth, they pumped and worked on him for what felt like a lifetime. I cannot put into words how traumatic this was to witness. This went on for five minutes, Chandler's body not responding with no heartbeat, no pulse.
Everyone was doing everything they could as we waited for paramedics to come. After the long minutes of CPR, the doctors placed the defibrillators on his chest to begin use. Just before using the defibrillator the two doctors yelled out they had a pulse.
Immediately after Chandler came back to us, he was ferried off the island by the medics and taken to a local hospital. The doctors at the hospital then monitored him for six hours. They told us that Chandler was a victim of something called "shallow-water drowning" or "shallow-water blackout." The prolonged lack of oxygen caused his brain to shut down without warning, and he went into immediate cardiac arrest.
We had never heard of shallow-water drowning.
The fatality rate of this kind of incident is over 90% because there are simply no signs of distress and nobody notices that the person has passed out. This specific type of drowning kills 4x faster than normal drowning. As you hold your breath for long periods of time you are not in pain or distress and it plays a trick on the mind. The effect is immediate when your lungs reach their limit.
When we hold our breath, oxygen levels in the bloodstream decrease. Usually the carbon dioxide rises in our body to a certain degree, telling us we need to breathe. But when you continuously and deliberately hold your breath, it keeps the carbon dioxide levels in the body extremely low, blocking the brain's signal to breathe. To put it simply, you black out before you realize what is happening.
Chandler does not remember anything about the incident.