I notice that the Drowning rules still make references to HP in a confusing and potentially frustrating way. I propose a simple revision to remove the references to HP and hopefully clarify the mechanism:
When the character finally fails her Constitution check, she begins to drown. In the first round, she falls unconscious (0 hp). In the following round, she drops to –1 hit points and is dying. In the third round, she drowns.
Unconscious characters must begin making Constitution checks immediately upon being submerged (or upon becoming unconscious if the character was conscious when submerged). Once she fails one of these checks, she immediately drops to –1 (or loses 1 additional hit point, if her total is below –1). On the following round, she drowns.
Change to:
When the character finally fails her Constitution check, she begins to drown. In the first round, she falls unconscious. In the following round, she gains the dying condition. In the third round, she drowns.
Unconscious characters must begin making Constitution checks immediately upon being submerged (or upon becoming unconscious if the character was conscious when submerged). Once she fails one of these checks, she immediately gains the dying condition. On the following round, she drowns.
Note that this writing has simply removed the references to HP from the text and has simplified it to a progression of worsening conditions that can clearly only be interrupted by outside forces (yes, for the purposes of my comment, the effects of a Contingency spell constitute outside forces). This also clarifies that the unconscious and dying conditions are not the result of standard HP loss and cannot therefore be "fixed" by casting healing magic; it also allows for the Paladin with 127 base HP and no wounds to be in the dying phase of this progression, have Water Breathing cast on her, and be at full health and ready to fight as the party swims around the reef without ambiguity, as opposed to perhaps being at some number of HP close to zero depending on how the GM reads the text...