Fresh water is fairly limited in Fallout 3. All the water sources in Vault 101 at the begining of the game aren't affected by radiation but other than that the only source of fresh water are randomly scattered bottles of purified water. These are all over but are commonly found in first aid kits hanging on walls. Purified water can also be found in all of the shelters associated with the radio towers scattered around the wasteland. If you find a tower, open your Pip-Boy and switch to the local map to find the entry to these shelters.