[My First Aid training tells me that] although brain death occurs in just a few minutes once the brain ceases to get oxygen supplied to it, if CPR is applied to a casualty, and the casualty can get in an ambulance and into a hospital within an hour or so (the "Golden Hour"), he or she stands a decent chance of surviving. The CPR would provide some oxygen to the brain and the rest of the body (while the ambulance makes its way to the scene), so if you can bring the casualty to shore within 2 or 3 minutes of them going unconscious, there's still reason for optimism.

As such, I would say it's not unreasonable to wait for the drowning person to lose consciousness if you aren't confident you could prevent or deal with him or her dragging you down.