From the Super Mario Wiki
Jumping Bloopers are jellyfish-like Bloopers that appear only in Super Mario Sunshine, and are found solely in Ricco Harbor. Jumping Bloopers occasionally jump out of the water, leaving them susceptible to being sprayed by F.L.U.D.D.. When sprayed, they land on the water and flatten. They can be used as platforms in this condition, and thus are useful in helping Mario cross toxic water. Mario must cross quickly however, as they will revert back to their normal form after about fifteen seconds. On a side note, they will never move in any direction horizontally and only jump up vertically.