Drinking during a meal of bread:
Some Poskim rule one is not Biblically obligated to recite Birchas Hamazon after eating [a satiating meal of bread] unless he drank [a liquid] during the meal. This however only applies if one was thirsty during the meal [and nevertheless did not drink; if however one was not thirsty, then he is Biblically obligated to recite Birchas Hamazon according to all opinions if he is satiated from bread, even if he did not drink at all during the meal]. [Furthermore, once one drinks a Revius, he is Biblically obligated to recite Birchas Hamazon even if he is still thirsty. Other Poskim however rule one is Biblically required to recite Birchas Hamazon after eating a satiating meal of bread even if he does not drink during the meal, even if he is thirsty.] Practically, we do not rule like the former opinion but rather like the latter opinion who holds that the Biblical obligation of Birchas Hamazon is dependent on eating to the point of satiation and not on drinking. Nevertheless, it is proper to suspect for the words of the former opinion. [Accordingly, if one is thirsty during the meal, he is to be particular to drink during the meal in order to be obligated in Birchas Hamazon even according to this opinion. This applies even if he only ate a Kezayis of bread. This applies even if he only has wine available as a drink, and by drinking during the meal he will not be able to recite Kos Shel Bracha over the wine. Nevertheless, if one does not desire to drink during the meal due to health reasons, he may avoid doing so.]
One is to initially drink a liquid during a meal of bread. This applies even if one only ate a Kezayis of bread. One may drink any liquid.
If one did not drink during the meal and is in doubt if he recited Birchas Hamazon, is he to repeat it?
Even if he is satiated from his meal and is thus required to recite Birchas Hamazon due to doubt, he is not to do so until he drinks [if he is thirsty].
 Admur 197/7; Rama 197/4
 Opinion in Admur 197/7; opinion in Rama 197/4; Opinion in Darkei Moshe 197/6; Opinion brought in Beis Yosef 197; Bach 197 [rules like Yireim]; Peri Chadash [rules like Yireim]; Yireim 253; Kol Bo 25; Mordechai Brachos 177; Shivlei Haleket 154; This opinion understands the opinion of Rebbe Meir in Brachos 49b to be Biblical
If one drank but did not eat to the point of satiation: According to the above opinion, if one ate a mere Kezayis and drank after being thirsty then he is Biblically obligated to recite Birchas Hamazon even if he is not satiated from the meal. [Rebbe Meir in Brachos ibid, as also explained in Tosafus Brachos ibid; Evident from Admur ibid towards end and M”A 197/12 and M”B 197/28 that according to this opinion one who drank but did not eat to satisfaction should do the Zimun over one who is satisfied but did not drink;]
 See Tehila Ledavid 1/57 that according to this opinion the obligation of drinking is only if he ate bread and not if he ate Mezonos,
 See M”A 174/2 that this includes if he drank wine
 The reason: As the verse [Devarim 8/10] states “Veachalta Vesavata” and the Sages [Rebbe Meir in Brachos 49b] expound this to mean as follows: “Veachalta” is referring to eating food and “Vesavata” is referring to drinking liquid. [Admur ibid]
 Admur ibid; Rama ibid; M”A 174/2; Yireim ibid; Poskim ibid; Olas Tamid 197/4; Elya Raba 197 end; Kaf Hachaim 197/32
 Olas Tamid states “Keshiru Shetiya” which seemingly means a Revius. Vetzaruch Iyun
 Olas Tamid 197/4; Elya Raba 197/10; Kaf Hachaim 197/32
 Tosafus Brachos ibid; Beis Yosef 197 argues on Yireim based on Tosafus ibid “and so is the ruling of all Poskim and so is the final ruling”; Michaber 197/4 omits former opinion; Erech Hashulchan 197/5
 Admur ibid “There words is not the Ikkur..the main ruling follows the…”; Beis Yosef 197; M”A 197/12; M”B 197/28 “The former opinion is a Daas Yachid”; Mamar Mordechai 197/12
The reason: As the above Drasha is a mere Asmachta that the Sages used to support the need to recite Birchas Hamazon after eating a Kezayis of bread, as they interoperated the verse “Veachalta” to mean eating food, and the measurement of eating is a Kezayis, whiles Vesavata is drinking. However, in no way was this Drasha intending to uproot the simple meaning of the verse which states that one is not obligated to recite Birchas Hamazon unless he ate and was satisfied, even if he did not drink. [Admur ibid; Tosafus Brachos ibid]
Other opinions: Some Poskim rule like the former opinion that one must drink if thirsty, in order to be required to recite Birchas Hamazon. [Bach ibid; Peri Chadash ibid]
 Admur and Rama ibid regarding initially being careful to have one who drank make the Zimun; Mamar Mordechai 197/12; Elya Raba 197 in end; P”M 197 M”Z 4; Kaf Hachaim 197/30
 M”A 174/2 based on Rama ibid who suspects for opinion of Yireim; Elya Raba 174; Beir Heiytiv 174/2; M”B 174/5; Ketzos Hashulchan 45 footnote 10; Teshuvos Vehanhagos 2/121; Piskeiy Teshuvos 184 footnote 86 and 197/4; This entire Halacha [of Michaber 174/3 and M”A ibid] is omitted in Admur 174 Vetzaruch Iyun
The reason: If one did not drink during the meal, and already recited Birchas Hamazon, and then drank afterwards, then according to the former opinion he now becomes obligated to recite Birchas Hamazon and was not Yotzei with the recital prior to drinking. To avoid this problem one should therefore always drink before Birchas Hmazon. [Teshuvos Vehanhagos ibid, brought in Piskeiy Teshuvos ibid; See Biur Halacha 184 and Sefas Emes brought in Piskeiy Teshuvos ibid]
 Poskim ibid do not differentiate in this matter
The reason: As according to the stringent opinion one is Biblically obligated to Bentch after eating a Kezayis and drinking, as explained above.
 M”A ibid; M”B ibid
Other opinions: Some Poskim rule that one is to always leave the cup of wine for after Birchas Hamzon in order to recite Kos Shel Bracha over it. [Michaber 174/3; Mamar Mordechai 174/3; Kaf Hachaim 174/9] Some Poskim rule that when a Zimun is present of three people, one is to leave the cup of wine for after Birchas Hamazon, as even according to Rama ibid it is only proper to drink during the meal and not required. [P”M 174 A”A 2]
 Az Nidbaru 10/22; Piskeiy Teshuvos ibid
 Birchas Habayis 17/9
 The reason: As according to some opinions he is not obligated to recite Birchas Hamazon if he did not drink, and hence even if he is satiated ikt is not a Safek Deoraisa in their opinion. [Birchas Habayis ibid footnote 13]