From the Super Mario Wiki
Krockhead Barrels are usually found hovering in the air. If Diddy or Dixie Kong touch the barrel, it summons several Krockheads, which the Kongs can use to cross large gaps. The Krockheads summoned by a Krockhead Barrel only surface above the water temporarily, meaning that the Kongs need to hop on them quickly, before they vanish back in the water.
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