And this is from Google Answers:
Subject: Re: how to build carnival games
From: murph-ga on 02 Aug 2002 12:17 PDT
Having just run a cheap, rented one for a day, i can give some advice
- it looked like i could build it.
Basically, You have a big 'Tank' That you're going to fill with water.
Plastic worked great for what i saw. If you can Cut a Plexiglas
window into the front of it then it will look really 'cool' because
you can see the person after they fall in. Now for the seat - it's
actually kinda simple. You have a board flat across one side of the
tank, held maybe two feet above the tank - it depends with what yours
will look like. Then you have a board (or 'seat) at right angles with
that one, Attached by hinges so that it can rotate from straight out
to up and down. You're going to want to put some kind of material over
this - something so the person won't slip. Now you attach a metal ring
to the Bottom of the 'Seat' On the far back. Nows the trick - have you
ever used 'vice grips' i believe there called.. See the bottom for a
link. You have it set so these pinch the ring, holding the seat
horizontal so someone can sit on the seat. If you have a good pair
it'll be able to hold 300 pounds without much trouble. Then you get a
long metal bar that goes through the vice grip so when it gets pushed
towards the 'front' of the tank it de-locks the vice grip and lets the
seat swing down and 'dunk' the person. You set it up so that if they
throw the ball or what not and it hits the target it's pushes one end
of the bar back, and like a lever the other side goes forwards and de
locks the pliers. Then all you have to do to reset it is push the seat
up, pull the bar back, and close the pliers.
PS - I know this isn't enough of a description to build it properly,
but hopefully it'll help.