Technically, you DID give him water...he just wasn't fast enough.
I once talked to Maggie while holding Billy Creel's head. When she whimpers about his death, she looks to the left. Looked like she was scared of confronting me and just said "someone" killed him. Afterwards I just left it at their door.
This reminds me of that question: Is it ironic if you smack a baby's face with a no more tears" shampoo?
I thougt it was amusing to jump up and down on a begger in third person just to see his reaction.. But I soon got bored with it before putting the poor guy out of his misery.. Bethesda should of included the option to let dogmeat attack the beggers or at least hump their leg until exhausted, just like most overly excited K.9's.. That would of been amusing to see the beggers reaction.. Personally I think the beggers are all working together, each one has his own collection of safety deposit boxes stuffed full of cash scattered carefully throughout the wastland. Or together they own a highly lucrative and successfull underground water shipping company. Perhaps we will never know. Never trust a begger.