Minecraft wiki entry for Slab, associated text: If a single slab is placed in a water source block, the empty half of that slab's block is waterlogged. If a slab is placed in flowing water, a pocket of air is created in the unfilled half of the block. If the player's head is in this pocket, the player can breathe and see as clearly as from an air block. If a single slab is placed in between two water sources or waterlogged blocks, the slab becomes waterlogged‌[Java Edition only]. This also happens when a water bucket or a bucket containing a fish is used on a single slab. Double slabs cannot be waterlogged.