Minecraft wiki entry for Man-made_lake, associated text: The water you see in your lake is superficial, what this means is that, contrarily to a regular pool of water one-block-deep, springs won't auto-regenerate. There's a set number of springs in the top layer of the lake, and if you fill your bucket with one of those, that spring won't be replaced naturally, you'll need to do it yourself by referring to the "There are currents!" section above.