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Depends. Are you using damage, drain, or penalty?
If I am reading the rule correctly, damage or penalty: no effect, drain: reduce the rounds by 2 per point.
Not really sure. I just read that Environment section because of another thread, and this drowning bit stuck out. Seems like any number of effects could reduce a stat and it's odd that this one isn't addressed directly in the description. Could also come up when a spell that increases Con ends while you are holding your breath.
I'm also unclear if "holding breath" is automatic, or if a character must be aware of the underwater situation and must actively hold their breath.

Tindalen |
"It depends on the will of Naderi. She won't let me drown."
Jared nailed it. As for the other question, I'd say holding your breath is automatic. Even an unconscious person can float face down in water for a minute before dying.
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.