“Medicine that boosts resistance. The effect lessens with each passing turn.” —In-game description, Shadow Dragon and Shin Monshō
The Pure Water (聖水 Seisui, Holy Water in fan translations) is a Stat booster that recurs across the Fire Emblem Series. When used, the Pure Water temporarily increases its user's Resistance by a set number of points that varies from game to game. This effect gradually wears off as the turn count of the battle at hand increases.
In the original games, the Pure Water is less common because individual inventories are limited, with most of the slots being reserved for carrying important weapons. With the onset of the newer entries to the series, individual inventory sizes have been expanded, creating more room for any extra items that may come in handy, one such item being the Pure Water.
For obvious reasons, the Pure Water should be used on units with low Resistance like Knights and Generals. It is still not recommended for them to be thrown into the fray of a battle where the bulk of the enemy army is comprised of magic users regardless, as the Resistance boost that they receive is transient.