Birkas Hamazon for water

 SEE RIDDLE HERE 

Parshas Hashavua עקב

ואכלת ושבעת וברכת    
BENTCHING ברכת המזון FOR DRINKING WATER

All he did was drink one glass of plain water
& therefore he needs to Bentch ברכת המזון.?      

     

According to Harav Hagaon R.Ch. Kanievsky Zt"l, If someone was thirsty at the end of his meal, and waited to drink water till after ברכת המזון, he must Bentch again ברכת המזון * (דאורייתא)   Based on the מרדכי who holds if you are thirsty at the end of the meal and Bentch then your Benching was only מדרבנן. Therefore, Reb Chaim holds, after you Bentched if you drink a glass of water you are now מחויב מן התורה  to Bentch(again) The first Bentching doesn't count since it was only a דרבנן Bentching. Mori V'Rabi Harav Hagaon R. Shlomo Miller Shlita  argues on Reb Chaim and holds there is no need to repeat the ברכת המזון after drinking the water.   *see Rema O.Ch. 197:4