May one drink water before Havdalah?
It is a Rabbinical prohibition to eat, drink, or even taste, any amount of food or liquid [other than water, as will be explained] prior to saying, or hearing, Havdalah over wine, or other valid beverages [if one did not begin a meal of bread prior to sunset]. This prohibition applies even if one recited Havdalah within Maariv Shemoneh Esrei. This, however, is with exception to water, which is allowed to be drunk prior to Havdalah [after nightfall/Tzeis Hakochavim]. [However, some Poskim argue and rule that even water is forbidden to be drunk prior to Havdalah, and thus the prohibition of drinking prior to Havdalah extends to all liquids without exception. Practically, the Chabad custom is to avoid drinking even water prior to Havdalah, starting from sunset. If, however, one is in the midst of a meal of bread, then he may drink water during the meal, just like any other food or drink, with exception to drawn water during Bein Hashmashos, as explained next.]
Drinking water after sunset, before nightfall: The above discussion of allowance of drinking water before Havdalah, only applies after nightfall. However, prior to nightfall, during the period of Bein Hashamshos, it is forbidden to drink water that was drawn at that time, even if one is in the midst of a meal due to Sakana. This matter will be elaborated on in a future Halacha IY”H.
From the letter of the law, it is permitted to drink water prior to Havdalah, however some are stringent and so is the Chabad custom. Even those who are lenient, are not to drink drawn water between sunset and nightfall, even if they are in the midst of a meal.
From the letter of the law may one drink water based soft drinks before Havdala?
It is questionable whether soft drinks that are water based are permitted to be drunk before Havdala just as is the law regarding water itself. [Practically the custom is to avoid drinking even water prior to Havdala, and thus certainly water based soft drinks are to be avoided.]
 Admur 299:1 “The Sages prohibited one from eating anything, or drinking any liquid other than water, on Motzei Shabbos until Havdalah is done over the cup, even if he said Havdalah during the Shemoneh Esrei, and even to taste a small amount is forbidden, due to the reason explained in 271:1.” Michaber 299:1; Pesachim 105a
Severity of eating prior to Havdala: Anyone who tastes anything before Havdala is liable for death through the illness of Askara. [Rebbe Akiva in Pesachim 105a] Askara is a form a strangulation in which the person cannot breathe [similar to an asthma attack]. The reason for this punishment is Mida Kineged Midah, as just as he satiated his throat in a prohibited way, so too his throat will strangle. [Tosafus Peachim ibid “Lo Kadi Amaya”; See Maggid Meisharim Vayeishev that the reason for this severity is because the Kelipos attach to the food at this time.
 If one began eating a meal [that consists of bread-Ketzos Hashulchan 94 footnote 3; Aruch Hashulchan 299:5] prior to sunset he may continue to eat and drink without limit even after nightfall. [Admur 299:2; Pesachim ibid]
 The reason: The reason for this prohibition is because one is to say Havdalah as soon as Shabbos takes leave, without delay. Thus, although in general the Sages only forbade a meal from being eaten prior to fulfilling a Mitzvah while snacking prior to the Mitzvah was allowed, nevertheless, here even merely snacking or tasting food was forbidden as Havdala is to be said close to the leave of Shabbos and thus even snacking will cause one to delay saying it within this time. [Admur 271:9; Kuntrus Achron 299:2; See also Admur 431:6 regarding Bedikas Chametz that it must be done immediately after nightfall]
 Admur ibid and 299:16; M”A 299:13; Ran Pesachim 21a
 See below!
 Admur ibid; Michaber 299:1; Rav Amram in name of Rav and Rabanan Divei Rav Ashi in Pesachim ibid; Rashi Pesachim ibid “And so is the Halacha”
The reason: As water is not considered a significant beverage to need to prohibit. However, prior to Kiddush it is forbidden to drink even water, due to the honor of Shabbos. [See Rashi Pesachim ibid; Aruch Hashulchan 299:1]
 Rav Huna in Pesachim ibid “I saw a man who drank water prior to Havdalah and I told him do you not fear from the death of Askara,” and so rule: Geonim in Shaareiy Teshuvah in end of Sefer Neharos Damesek 80 p. 6; Midrash Talpiyos “Havdalah” 4 p. 118 “It is forbidden to drink water before Havdalah and one who does so is liable for death”; Arizal in Peri Eitz Chaim Shaar 18:24 “My master severely warned me not to drink water before Havdalah as one who drinks water by Bein Hashamshos of Motzei Shabbos can die of Askara”; Yalkut Reuveini; Likkutei Maharich; Kaf Hachaim 291:16 and 299:6; Ketzos Hashulchan 94 footnote 1
 Hayom Yom 3rd Iyar; Sefer Haminhagim p. 68; Ketzos Hashulchan 94 footnote 1 based on Arizal.
 Drinking water on Shabbos after sunset, prior to nightfall: Prior to nightfall, during the period of Bein Hashamshos, it is forbidden to drink drawn water due to Sakana, even if one is in the midst of a meal. [See Admur 291:2; Opinion in Rama 291:2; See Kaf Hachaim 291:16-17; Piskeiy Teshuvos 291:3] See Rama ibid for a number of opinions on this matter; See Admur ibid that we are stringent although that this prohibition only applies to drawn water; See Admur ibid and Kaf Hachaim and Piskeiy Teshuvos ibid that it only applies by Bein Hashmashos and not after nightfall.
 See Piskeiy Teshuvos 299:1 footnote 6