This was inspired by a combination of Dr. Bob's and the equally esteemed Maxwell Buchananan's annotations on the linked idea.
The water-pogo apparatus consists of a long, stiff rod (no giggling at the back, please) - perhaps 10-20m of your finest carbon fibre. Atop this rod (for vertical it is) is a securely-affixed chair equipped with a safety harness.
At the bottom of the pole is a hollow cigar-shaped tank, looking rather like a fat spearhead on the end of the rod. The tank has a displacement of maybe a tonne (of water), but weighs only a fraction of this.
The Water Pogoer (or indeed anyone who wants to go pogo) is strapped into the chair, and released from a great (but carefully calculated) height over a deep body of water.
They plummet to earth and, with any luck (and some aerodynamics) the pole remains upright, causing the hollow tip-tank to pierce the water at an impressive speed. Its streamlined shape means that it enters the water without sending a massive impact through the pole, and it continues to descend rapidly below the waves, slowed by a combination of hydrodynamic drag (hopefully modest) and buoyancy (hopefully large).
If the calculations have been done right, the weight and momentum of the Water Pogoer will carry the tip-tank down and down, until it finally comes to a halt when the pogoer's chair is only inches from the water surface.
We now have an interesting situation. What we have is a man sitting on a chair, stationary at this instant, a few inches above the water surface. Below the chair is the pole and, at the far end of the pole many metres below the waves is a hollow tank with a buoyancy of about a tonne.
The events of the next few seconds should be fairly self-evident. The exact height to which the Water Pogoer will be catapulted by the violently-breaching tip-tank will depend on things like drag and mass, but he and his pole should clear the water by a quite impressive margin, allowing the entire spectacle to be repeated again and again, at ever diminishing heights.-- MaxwellBuchanan, Mar 22 2011 Unspired by: Daredevil_20Sky_20Diving_20Stunt [MaxwellBuchanan, Mar 22 2011] //the esteemed Maxwell Buchananan// Such modesty! They do say that modesty is it's own reward. 'They' are, of course. wrong. I prefer croissants myself! +-- DrBob, Mar 22 2011 I'll cut you in for 40% of all baked goods.-- MaxwellBuchanan, Mar 22 2011 random, halfbakery