Aqua Pura is literally "pure water" (in Latin) that is available once the add-onBroken Steel is installed, and the quest Take it Back! is completed. This water is from the Potomac river basin, and has been completely cleansed of its radioactivity and contaminants by the purifier at the Jefferson Memorial.
Drinking the water after choosing to infect it with the modified FEV will, along with restoring 20 Hit Points, result in a player debuff and upon consuming three bottles; it kills the player character. The debuff lowers Intelligence by one for 24 minutes and lowers Strength; first by five points for 12 minutes, then by four for 6 minutes, then by two for a final 6 minutes (for a total of 24 minutes).
Why the Infected Aqua Pura kills the player character can be explained by the fact that the Lone Wanderer was actually born outside of a Vault and thus they are not immune to the modified FEV, which was designed to kill anyone who had prolonged exposure to the radiation and other mutational effects of the Wasteland.
Other than the value and the label, uninfected Aqua Pura is identical to the purified water found throughout the Capital Wasteland prior to the events portrayed in Broken Steel. Notably, this water is the same color as dirty water, presumably because it was dirty water prior to purification.
pcps3xbox360 Giving any amount of aqua pura to water beggars will result in their death at the next map change, regardless of whether or not the Aqua Pura contains Modified FEV. [verified]
pcps3xbox360 Sometimes after drinking aqua pura a message will appear saying that "aqua pura has worn off". This is like when a chem wears off but there were no effects to begin with. This message may appear at random times, days after drinking Aqua Pura. It's not really a bug but a scripting quirk. Because tainted Aqua Pura has a debuff effect it's treated like a chem. The taint effect is still applied to the player, but since the effect fails a condition check, the actual debuff and player's death chance aren't applied. The pure Aqua Pura is thus also treated as a chem though it has no ill effects; the end result being the "worn off" messages. [verified]
xbox360 Non-player characters use this as an aid item during firefights. It has the same effects on them if there is Modified FEV added to the water. However, non-player characters drinking it will trigger the script that attributes the negative effect to you. In essence, if non-player characters drink many infected aqua puras, the script kills the player character instead of them. [verified]