Do Ash Blocks absorb water, or is that just me? Or is it because of the Underworld?
All water evaporates in the Underworld. However, due to the often laggy nature of big bodies of water, sometimes you can glitch a puddle into the Underworld without it evaporating. However, once it settles, if you mine a block from the bottom of that puddle, it will begin to evaporate.