Hi,

can be played Crushing Block when an hero enters a water space in SoB?

for Water I mean Sea Space not usual water spaces.

My gut instinct is that the water itself counts as a terrain type (like mud) and thus the space is not considered "empty." I could very well be wrong though.

Steve-O said:

I agree. Though water has not been properly defined, it is clearly not an 'empty' space, any more than a tree space is.
Corrupted Terrain has been defined as 'empty', so water could be later defined that way, but until such a ruling happens, it isn't that way currently.

So we are fairly sure that no, you cannot play a crushing block on a water space.