I stumbled across the Instinctive Drowning Response online in recent years, decades after nearly losing my life as a young child. The description was exactly like my memory of events, and I can say from experience, being surrounded by people and even a lifeguard within feet of me, I couldn't make a sound, or control my limbs.
It was like paralysis. I couldn't have grabbed anything even though my life DID depend on it, because my hands were locked, elbows bent, fingers spread wide, spasming like a frog, trying to push down the surface of the water (which doesn't work at all, but somehow the body won't respond to any attempts to control it, by the mind, in this state).
As my head tipped back, I thought to myself (but it felt at the time like a voice telling me something) "this is it: no one else can save you, so you have to save yourself now, or you will die" and I took that opportunity of my head tipping back, to gasp for air, and then let my head relax back while focusing on becoming like a jellyfish, letting my chest be what floated, never quite exhaling all the way, and timing breathing with when my lips could break the surface.
It worked. Eventually I could move my limbs weakly, enough to start slowly drifting toward the pool edge, still floating chest-up and breathing when I had the chance, til I could lay a hand on it. Rested like that, til I regained full control of my limbs.
So if I found someone in the Instinctive Drowning Response,
I would essentially try to coach them through the steps I took to saving myself in the same situation, while also calling responders and looking for something in case they could actually grab it.