I'm not sure if this is a reported issue or not tried searching for it and couldn't find it. When placing sponges to clear out water they don't retain the water I guess is what you would call it. You can place a line of sponges and by the time you get down to the end of your line you turn around the water is filling back up. I'm not sure if it's the water souces that are left that makes it fill back up but something isn't working correctly.
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Dry sponges absorb water source blocks around them when placed, becoming wet. Wet sponge blocks do not block water from flowing back over and around them, and they do not prevent new water sources from forming according to normal water mechanics..
You can learn more about using sponges and water spreading on the wiki.
Sponges can absorb limited amount of water. If water pool is too large, water will be reform from water sources that remain after sponge is placed. You need to divide your water pool into smaller sections (i.e. create 5x5 squares using sand of gravel to create walls) before trying to drain them.