FOR DRINKING WATER
& therefore he needs to Bentch ברכת המזון.?
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)
ברכת המזון
The first Bentching doesn't count since it was only a דרבנן Bentching.
Reb Shlomo Miller Shlita