Does Wielder of Water get its bonus if it starts combat with another unit, but that unit is killed by Hidden Blade during showdown?

Ruling: Yes, Wielder of Water will proc. Wielder of Water's ability is a permanent ability that checks if he is alone at the location, regardless of how combat started.

Nuances:

• It doesn't matter whether Wielder of Water started combat alone or became alone during combat; the ability checks the current state