Geyser
Geysers are found in every cave in Pikmin 2, always on the final sublevel. Their purpose is as a means of returning to the surface: Olimar, Louie or The President, and the Pikmin jump into the water, which lifts them upwards through a hole in the ceiling and they land at the landing site of the area the cave is found in. Some longer caves also have them on so-called "rest" sublevels, generally a few floors down, as a means of leaving if you are lacking in Pikmin. Rest levels often also feature eggs and Candypop Buds.
Pikmin
In the first game, geysers are small bumps in the ground. They have to be unclogged before they are usable, this can be done with any Pikmin species. However, the captain cannot do this himself. They eject water that can shoot Olimar into the air, allowing him to reach places otherwise inaccessible or to create shortcuts over small elevations.