Water Stone
From the Super Mario Wiki
The item, the Water Stone.
The Water Stone is an object that can provide an endless stream of water. In Paper Mario, two Lakitus stole it to increase the Puff-Puff Machine's power. One of them accidentally dropped it near Rosie, who claimed it as her own. The stone was originally from Lily's pond, which gave her all the water she needed to live. Rosie will keep the stone until Mario can find something more beautiful.
