Close to the end of the third act, Sonic will find Robotnik sitting in his hovering Egg Mobile, which will begin to fly into the depths of Tidal Tempest. Similar to the boss of Labyrinth Zone, The player can hit Robotnik four times before he departs into the sea. After chasing the doctor, the player finds the doctor floating in the water, with a shield of bubbles around him. Robotnik will partially dive towards the player while his bubble shield spins, and will fire a few projectiles at the ground. Sonic, however, has an advantage here, as the bubbles are the main oxygen supply to survive underwater, and once a bubble is gulped it will leave one less bubble around the Egg Mobile.
Once most or all of the bubbles are gulped, the player only needs to hit Robotnik once more to defeat him, and then the water level will drop through the floor.