A dunk tank, also known as a dunking booth or dunking machine, is an attraction mainly used in funfair

Funfair

A funfair or simply "fair" is a small to medium sized travelling show primarily composed of stalls and other amusements. Larger fairs such as the permanent fairs of cities and seaside resorts might be called a fairground, although technically this should refer to the land where a fair is...

s, fundraisers

Fundraising

Fundraising or fund raising is the process of soliciting and gathering voluntary contributions as money or other resources, by requesting donations from individuals, businesses, charitable foundations, or governmental agencies...

, and personal parties

Party

A party is a gathering of people who have been invited by a host for the purposes of socializing, conversation, or recreation. A party will typically feature food and beverages, and often music and dancing as well....

. Basically, a dunk tank consists of a large tank of water, over which a seat is suspended. By striking a target, the seat will tip or fall into the tank of water, thus "dunking" whoever is sitting on the seat.
There are many types and brands of dunk tanks commercially available.