Do I need to know how to swim to do shallow water blackout?
No, from my understanding it only requires
extremely shallow water, like a bathtub or a paddling pool, or in extreme examples even a puddle. It's
not like 'traditional' drowning where the water is too deep for you to stand and so eventual tiredness or an inability to swim mean you sink below the water and drown through lack of oxygen.
The threads on this method should explain the mechanism more clearly.