Gemara

(Beraisa): They would talk with him from when he entered [the Lishkah to take Shekalim] until he left [lest he hide some in his mouth].

Question: He could fill his mouth with water!

Answer (R. Tanchuma): He cannot, due to the Berachah.

Eruvin 30a (Mishnah): If one vowed not to eat Mazon, he is permitted water [and salt].

(Rav Huna): The Mishnah discusses one who forbade 'anything that is Zan' (satiates).

Inference: Water and salt do not satiate, but everything else does.

Berachos 41b - Question: Why do things eaten due to the meal during the meal not require a blessing beforehand or afterwards?

Answer (Ben Zoma): [The blessing on] bread exempts them.

Question: If so, bread should exempt also wine!

Answer (Ben Zoma): Wine is different, for it causes its own Berachah.

Rishonim

Rosh (Berachos 6:29): Since one cannot eat without drinking, bread exempts all drinks. Bahag says to bless on water during a meal. R. Yom Tov says that we asked that bread should exempt wine, for wine sustains, but water does not sustain, like we say [in Eruvin - Ma'adanei Yom Tov 7], that one who vowed from Mazon is permitted water. I disagree. Wine exempts all other drinks, even though Borei Peri ha'Gafen does not apply to them, because wine is the head of all drinks, so it is primary regarding Berachah, and all are Tefel (secondary) to it. Similarly, bread is primary to the meal, and the drinks are only due to it, so it exempts them. To avoid the Safek, one can sit where he will eat and bless with intent to drink during the meal.

R. Yonah (Berachos 29b DH umi'Kan): R. Tam learned from here that one does not bless on water in a meal. Water does not cause its own Berachah. We bless on it sheha'Kol, the all-inclusive Berachah. Therefore, it follows the bread, and one does not bless on it. Some say that we bless on it. Wine is like food. Bread would exempt it, if not that wine causes its own Berachah. Water is unlike food, so bread does not exempt it. This is wrong. Also water, one drinks it due to the food. Even if one was thirsty from the beginning, we do not say that the water is not due to the food, so he must bless on it. Rather, since he did not want to drink then (before eating), lest it harm him, he drinks due to the bread, so bread exempts it.

Hagahos Maimoniyos (Hilchos Berachos 4:9): In France the custom is like Ri and R. Tam, not to bless on any drink during the meal.

Rashba (41b DH Iy): The Gemara asked that also wine is due to the meal, for it is in order to soak the food. Rashi explains that wine causes its own Berachah, i.e. in several places we bless on it even without need to drink it (e.g. Kidush). If so, water during a meal does not need a Berachah. Also it (like wine) is due to the meal, to soak the food. This was the custom of Rashi, R. Tam and the great Chachamim of France [not to bless on water in a meal]. Some explain that wine causes its own Berachah, i.e. drinking causes its own Berachah, for eating and drinking are two separate matters, and one cannot do them at once. The Ra'avad says that if so, one must bless even on water [during the meal]. A support for this is 'one who drinks water for thirst must bless. This excludes one who was choking on a bone.' (Berachos 44b) This connotes that other cases, even amidst eating, require a Berachah. I disagree. The Gemara did not learn from there to drinking during the meal, for one drinks during the meal due to thirst, for the eating makes him thirsty! Even if one need not bless during the meal, we cannot learn from there. Rather, one need not bless because the bread exempts it. The water is Tefel to the bread. The opinion of Rashi and R. Tam is primary.

Poskim

Shulchan Aruch (OC 174:7): If one does not have wine, and he drinks water or other drinks during a meal, he does not bless on them. They are considered to be due to the meal, for it is not normal to eat without drinking. Even wine should not require a Berachah, just because it is important, it has its own Berachah. Other drinks are not important, and do not require a Berachah.

Beis Yosef (DH v'Al): Tosfos (41b DH Iy), R. Yonah (29b DH Yayin) and the Mordechai (136) brought the opinions in the Rashba, and conclude like Rashi and R. Tam. Also Semag and Semak say so.

Mishnah Berurah (30): We do not refrain from bread [until Birkas ha'Mazon], therefore everything is considered in a meal, even after he finished eating.

Shulchan Aruch (ibid.): Even if he was thirsty before the meal, since he did not want to drink then lest it harm him, he drinks due to the bread, so bread exempts it.

Magen Avraham (12): This is only if he eats a fixed meal. If he has only a little bread, see 212:1. (If he eats a little to sweeten the drink, the drink is primary.)

Shulchan Aruch (ibid.): Some say to bless on water during a meal.

Beis Yosef (DH v'Hagahos): The Agur brings from Or Zaru'a that the custom is to bless on all drinks during the meal. One who follows the Ri and R. Tam and blesses only on wine does not lose. This is proper.

Mishnah Berurah (32): All agree that Birkas ha'Mazon exempts the Berachah Acharonah.

Shulchan Aruch (ibid.): One who wants to avoid the Safek should sit where he will eat and bless with intent to drink during a meal.

Beis Yosef (DH veha'Rosh): The Rosh favors those who say not to bless, but advises to bless before the meal with intent to drink during the meal. Hagahos Maimoniyos says that Maharam used to do so.

Taz (9): Why doesn't he need a Berachah Acharonah after drinking a Revi'is before the meal? Birkas ha'Mazon does not exempt Borei Nefashos or me'Ein Shalosh (177:2)! Here is different, for what he drinks before is needed for what he will eat during the meal. This is why Birkas ha'Mazon exempts wine drunk before the meal. If one was already obligated a Berachah Acharonah, Birkas ha'Mazon does not exempt it. It says in Kavanos ha'Ari Zal 'one should drink less than a Revi'is before the meal, and bless before and after, and intend to exempt what he drinks during the meal. Once, I drank more than a Revi'is, and they showed me on my forehead that it was a Berachah l'Vatalah.' He obligates a Berachah Acharonah on what one drinks before the meal, unlike the Rosh. A Berachah Acharonah would interrupt between the Berachah before and what he drinks during the meal! Why was he shown that he blessed l'Vatalah? He omitted a Berachah! If he said Borei Nefashos before the meal, he needed to bless sheha'Kol on water during the meal. If he did not say Borei Nefashos, he omitted this Berachah! Also, why did he [normally] bless before and after? We do not bless after less than a Revi'is! One may not rely on this. It is not really from the Ari Zal.

Kaf ha'Chayim (47): R. Chaim Vital says 'one should bless before the meal and drink less than a Revi'is. (It seems that this should say less than k'Zayis, for some obligate a Berachah on a k'Zayis.) Once, I drank a Revi'is, and the Ari Zal looked at my forehead and said that I drank without a Berachah Acharonah.' The Taz saw an errant version of this. This teaches that if one drank a Revi'is before the meal, Birkas ha'Mazon does not exempt it, for it was seen that R. Chaim Vital omitted the Berachah Acharonah.

Magen Avraham (15): Berachos Ramam (R. Moshe Mintz) says to ask one who is not eating to bless, and be Yotzei through him.

Bi'ur Halachah (DH va'Yevarech): If he does not enjoy the water, he should not bless on it.

Rema: The custom is like the first opinion.

Mishnah Berurah (39): The Chayei Adam says that coffee after the meal is to digest the food, so one blesses on it. It is proper to bless on sugar and exempt the coffee. There is no need on Shabbos or Yom Tov, since Borei Peri ha'Gafen [or sheha'Kol on beer] of Kidush exempts also this.

Kaf ha'Chayim (49): If one does not have sugar, he thinks every word of the Berachah in his heart. Some say that one is Yotzei through this.

Bi'ur Halachah (DH veha'Minhag): If he has wine, it is good to bless on it during the meal and exempt all drinks.

Kaf ha'Chayim (50): One should be stringent like the Shulchan Aruch.

Chacham Tzvi (121): R. Tanchuma answered that he could not fill his mouth with water, due to the Berachah. Yefe Mar'eh explains that perhaps he will swallow water, and transgress benefiting without a Berachah. This is astounding. He could bless beforehand with intent for the water that he will swallow! Also, a Mishnah teaches that one who drinks for thirst blesses sheha'Kol. The Gemara explains that this excludes one who was choking. One who drinks not due to thirst, rather, due to Ones, need not bless at all, since he has no Hana'ah, and water does not sustain. The Gemara explains that this excludes one who was choking. All the more so, one who did not intend to swallow need not bless! All the more so, we need not refrain from filling his mouth with water due to a Safek lest he swallow! Rather, the Gemara means that one who takes Terumah from the Lishkah must bless [on this Mitzvah], therefore he cannot fill his mouth.