From the Super Mario Wiki
The Snorkel Snake, also called "Snorkle Fish", is an enemy character featured in Yoshi's Story. It prowls only one area of the stage Lots O'Jelly Fish, waiting for an unsuspecting Yoshi to attack. When it spots a Yoshi, it would swim in circles around him. However, its body is not long enough for it to make a complete circle, and there will always be an opening for the Yoshi to quickly swim out of. The Snorkel Snake is seen swimming on the world map of the Sea.
It resembles a large, white serpent with green spikes and a large fin for a tail. Its head is a large ball with red eyes, flippers, and a spout for a mouth. It is invincible since Yoshis can't throw eggs underwater, and the spikes on its body are harmful.
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