Hi. When i fight with the wither boss, on normal world, on the water, i can break the water blocks! I can't belive at this. I can break te bloco and i listen a sound together.
MC-1985
Wither Boss
Resolved
Invalid
0
Pretty sure it's intended behaviour, they do destroy water blocks close to them.
On a side note, check this video of DocM, where he shows a way of drowning the Wither. He breaks the water around him, but not the block above.
http://www.youtube.com/watch?v=QeB0YJ--aQU (around 16:00)
If this is not your case, please add a screenshot.