I’ll give you Megaton, they do explicitly have a water purifier. I guess we can assume Rivet City has a water purifier, though we don’t see one iirc. They might’ve been using Jefferson Memorial as a water purifier/desalination plant, but it seems to have been taken over by Super Mutants since the Brotherhood abandoned it around the time the Lone Wanderer was born. As for food though, I remember dialogue in Rivet City that specifically says that every once in a while some dumbass will try to eat a fish from the river and they’ll get really sick from it, so I don’t think they’re getting food from the ocean if we’re expected to believe that line of dialogue.
Also, I don’t doubt that Capital Wastelanders have plenty of sources of meat, but humans need plants to survive. The only plant-based foods I remember seeing is the hydroponic farm in Rivet City (and maybe those potted plants on the deck), and it doesn’t seem like that could provide enough nutrition for everyone in the wasteland.
Really, I think we’re supposed to believe that everyone has been surviving off pre-war food for two centuries.