They are the aquatic ancestors of the Jellybacks. When a customer bumps into a Jellyblurp under the water, they will bounce off the Jellyblurp's gelatinous body.
Just like Jellybacks, they cannot be destroyed regardless of the Swim Boost skill, so the player must swim past them.
Multiple ones in the water can be an obstacle, such as in X-Zone, as they will bounce the player back into a wall or into a baddie. They are not as useful as the Jellybacks, because you don't need to bounce underwater.