Ok, the Vault Wiki says that Endurance modifies the amount of time it takes for your H²O bar to be empty. I was wondering, does anybody know where this is located in the GECK?

Its just a guess, but look up the game settings under Gameplay -->Settings and search for "swimbreath". I think either increasing the fActorSwimBreathBase or decreasing the fActorSwimBreathDamage would have the result of increasing the time underwater before drowning.