The Bubble Shield (also known as the Water Shield or Aqua Barrier) is a power-up item from Sonic the Hedgehog 3, as well as its locked-on form, Sonic 3 & Knuckles. It is obtained either by destroying an item box with a water droplet symbol (pictured), or by collecting a blue gumball in the Bonus Stage.
This shield is the only one of the three obtainable shields that can be used underwater. It absorbs one hit as well as allowing the player's character to breathe underwater. It can also deflect shots from small badniks, such as the Blastoids from Hydrocity Zone.
Sonic can use this shield to bounce and slam downwards, slightly increasing his jump height, but if done on top of a badnik, the player can achieve a vast jumping increase, especially if done in water.
The bubble shield appears in Sonic Generations for a special mission in Chemical Plant, in which it is called the Aqua Shield. After completing the mission, the shield can be bought as a Skill for Classic Sonic at the Skill Shop. The shield costs 300 skill points to buy and 70 skill points to equip. The Bubble Shield retains all the abilities it had in Sonic 3 & Knuckles.