So it would seem:
The Witch was too much afraid of the dark to dare go in Dorothy's room at night to take the shoes, and her dread of water was greater than her fear of the dark, so she never came near when Dorothy was bathing. Indeed, the old Witch never touched water, nor ever let water touch her in any way.
She also always carried an umbrella, presumably to avoid getting wet:
The girl had to work hard during the day, and often the Witch threatened to beat her with the same old umbrella she always carried in her hand.
As in Project Gutenberg's Wizard of Oz, in Chapter 12 - The Search for the Wicked Witch.
