Minecraft wiki entry for Fluid, associated text: Generated structures never cause block updates to adjacent fluids when they generate. For example, a cave entrance that is created partly below water level at the edge of a body of water or lava does not cause the fluid to flow until it receives a block update. On the other hand, fluids created as part of a structure flow immediately if not completely confined; this includes holes in the bottom of an ocean that open into a ravine below.