"A newly developed sub-aquatic Seal-type robot. Its breath can freeze the water around them in an instant, creating obstacles when Sonic needs to look for air bubbles."
Steelions attack by emitting some kind of freezing pulse from their mouths, thereby freezing a small area in front of them as well as the Steelion itself. The main purpose of this attack is to create an obstacle for the player, cutting off shortcuts and generally slowing down the player so they ultimately drown. If the player is in the vicinity of the pulse when it's performed, it will cause damage to the player.
Steelions tend to perform their attack as soon as the player approaches, making them hard to defeat.