The Full Water Can is an item that is given to the player during the Water Can Thief (HT0002) quest. The Full Water Can is acquired by using an Empty Water Can at the Water Pump. It is used in gardening, where the player has to use the water can on a plant once a day to grow it. But, after 1 use, the Full Water Can becomes empty again. So, to water all the plants, if the number of plants planted is more than 1, the player must go back and forth from the planter boxes and the pump.
When the player has bought an Extra Water Can, all of the cans are filled at the same time when using the Water Pump, which allows to water more then one plant.