Otamamon is an Amphibian Digimon protected by Divermon. It is Rookie level.
When Divermon thought Takato, Henry and Terriermon would attack Otamamon, they had to fight it and beat it. After that, Divermon said he was only protecting Otamamon. And when they asked about the route to get out of the water and Divermon showed them one tube, Otamamon's bubbles protected them from electric shocks in the tube.