From the Super Mario Wiki
The Snorkel Snake is an enemy character featured in Yoshi's Story. It would prowl the Yoshi Sea, 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. It's possible to see the Snorkel Snake swimming around the levels on the world map.
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 Yoshi can't throw eggs underwater.
It is unknown if Snorkel Snakes are a species. Oddly, the Snorkel Snake's spout can't hurt any of the Baby Yoshis, but all the other body parts can.