Try using a large, metal washtub, 1/2 full of water.
Forcefully, smack the water square on with a simple
toilet plunger. (Don't drop or throw it -- plunge!)

For a deeper sound, a garbage can may be
substituted, also 1/2 full. (This is to assure you get
the nice, echo-ey "hollow" sound)
Attach a long broomstick to wide, flat board cut to fit
inside your garbage can easily without banging the
sides.
Plunge the device forcefully and stand back so you
don't get splashed.

You may need to set up an off-stage microphone to
fully benefit from the sound. But if you're going to do
that, there are plenty of sound effects CD's available
that may have the sound you're after.