Only way for this to work is if you are by an ocean.
Ocean = opposite of shallow water
SWB can theoretically happen in any level of water that covers your mouth and nose. But it's very difficult to do purposely, because you have to lose consciousness while your mouth and nose are submerged, and then stay submerged while unconscious until it's (permanent) lights out.
SWB happens to plenty of people who aren't doing it on purpose- I knew someone who was just joking around in a pool and doing the "how long can you hold your breath" thing over and over, he lost consciousness and had to be pulled out of the pool, ending up with severe medical consequences.
But each person's body is different as to the amount of hyperventilation and/or breath holding they need for unconscious to occur. It's one of those ways of dying that seems easy because it happens to people on accident, but it still requires a specific sequence of events which are hard to carry out purposefully.