Minecraft wiki entry for Underwater_home, associated text: To make an air pocket in Bedrock Edition, place a solid block that can break quickly, like slime block or tnt. After breaking the block, quickly place a block that displaces flowing water, like a button, sign, or trapdoor, before a water source block forms in that space. The block where you place the sign is counted as a flowing water block, so a sign can create an air pocket there.