The Water Shield, Aqua Shield, or Bubble Shield is a power-up introduced in Sonic 3, and a variant of the standard Shield. In addition to serving as a one-shot barrier against damage and life loss, the Water Shield prevents drowning, allowing characters to stroll along underwater for as long as they want without fear of the dreaded drowning music. It is also capable of repelling certain badnik-fired projectiles, such as the Bloominator's spike balls and Blastoid's bullets.
When playing as Sonic, holding a Water Shield exchanges the Insta-Shield move with a Bounce Attack, allowing the hedgehog to barrel downwards on top of enemies, and springing back up higher than normal jump height.
The Water Shield's next appearence is in Sonic R. Here it has none of the same functions as the Sonic 3 version, instead allowing its holder to walk across the surface of a body of water without sinking. It is, however, one-shot; on returning to dry land, the shield evaporates.
It returns once more in a few Sonic Generations missions with a true return to form, allowing bubble bounces and the ability to breathe underwater.
The Water Shield appears in iOS/Android remake of the original game if you set "Items" to "S1+S3" or "S2+S3" in the Level Select menu (or place a Water Shield monitor in debug mode). It behaves exactly as it does in Sonic 3 & Knuckles, including the bounce attack.