This was well wrangled over in the oughties. The GTMO version of this is more psychological than physical, involving a cloth which blocks any of the water from getting into the “torturee’s” mouth or nose. IIRC in WWII the Axis did not bother with the cloth, it was literally an open air drowning.
The purpose of waterboarding is to simply trigger the anti-drowning reflex. The purpose of drowning is to kill someone.
If your intent is to acquire information, waterboarding is vastly superior to drowning.