The Propeller Shoes can only be found inside Monitors in Tidal Plant Zone. When picking up this power-up as Sonic, the player can move around quickly in all eight basic directions with the while in underwater sections for eight seconds. Crashing into anything hazardous will make the Propeller Shoes disappear though.
The Propeller Shoes are basically the equivalent to Tails' Sea Fox which is also used for underwater exploration.
The controls of the Propeller Shoes are similar to the Rocket Shoes, which let the player move in all eight directions in midair.