The Sea Monster is very similar to an eel, except it is a couple of meters tall and several meters long. It has gray, swirly skin with green-glowing fins, blue eyes, a large mouth with sharp teeth and a short and stubby horn over each eyebrow.
The Sea Monster only appear towards the end of Aquarium Park Act 6, in the underwater sections where they reside inside large caves. If the player gets too close in front of their cave, the Sea Monsters will stick its head out for a couple of seconds, before shooting straight out to attack Sonic, causing damage upon contact. They are virtually invulnerable to attack, so the only option for the player is either avoid them or wear protection.
- The Sea Monsters are similar to the Orcas from both Sonic Adventure and Sonic the Hedgehog (2006), as they are both non-anthropomorphic that serve as enemies to the player.