Withers can't break slabs and stairs in water.

 How to get this bug:

1.build 15*15*15 crying obsidian cube with 5*5*5 empty cube in centre.

2. Add slabs in water at top and under cube. 

3. Add stairs in water as in video 

4. Summon wither

5. Spawn mobs that can't hurt wither (I used pigs)

6. If nothing hurts wither, he can't go away from cube.

https://youtu.be/UBBtMpv9hJw