From the Super Mario Wiki
Watering Holes, also known as Fountains, are small fountains in Mario & Luigi: Superstar Saga. They appear throughout the Beanbean Kingdom, and premiere at Hoohoo Mountain. They consistently spew water into a small basin, allowing Mario to swallow some and turn into Pump Mario. If Luigi attempts to swallow some water, he is unable to hold most of it in and immediately spits it out. Luigi can instead use his hammer, High Jump, or Thunderhand on Mario while he is carrying water to eject it in a variety of ways. A couple of Watering Holes start off empty, and require a specific ! Block to be pressed for them to activate.