ducking stool

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A chair attached to a board or pole, formerly used as a punishment for offenders, in which a person was tied and ducked into water.

from The Century Dictionary.

• noun A stool or chair in which common scolds were formerly tied and plunged into water.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun historical A chair used to punish women, especially scolds, by ducking them in water.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an instrument of punishment consisting of a chair in which offenders were ducked in water