As is evidenced by the variety of posts, there are a number of different ways to approach this problem (some of which I can think of that haven’t been mentioned yet).
Part of the problem with water is the high surface tension it has. That’s what makes it act like concrete at high velocities. In theory, a non-flat water surface (Jacuzzi theory) should make for a “softer” landing than a flat water surface. Also, the addition of a surfactant to reduce the surface tension could help. However, at terminal velocity, I’m guessing neither of these would be enough to do the trick.
A previous poster mentioned density as being important, but I believe viscosity is really the property that would most affect a fluid’s ability to reduce your velocity. Ideally, the pit would contain layers of fluids of increasing viscosities from air to, say, water. This would be possible if the fluids were immiscible and had increasing density from the top to bottom. Unfortunately, most fluids meeting these requirements are flammable and/or toxic … not good for jumping into.
I also like the ramp idea, but agree with many of the limitations mentioned. An alternative might be to have a funnel (also mentioned previously) going down into a tube. The tube could then gradually decrease in angle from vertical to a cork screw. It may even make sense to have water flowing along the surface of the tube in a vortex (picture a water slide). That would decrease the heat of friction and decrease your velocity while maintaining an airway for breathing. Once your speed was sufficiently reduced, you could end up launching horizontally into a pool of water (again, picture the pool at the end of a water slide) at the bottom of the pit.
The fan idea someone mentioned is interesting. However, I’d suggest that just as a method just to slow you down, with another method used for the actual landing. It seems like it would be difficult to be able to adjust the airspeed with enough precision to allow you to land on your feet (especially considering that terminal velocity can change drastically depending on your orientation … if you went from a skydiver fall position to a feet-down position for landing, you would start to pick up speed again).
However, I think the easiest solution could be an air pillow (such as that which is used by stunt men), or a series of air pillows of increasing resistance. The degree to which the air is restricted from escaping has an effect equivalent to increasing the viscosity of the air. With a series of air pillows, the first might “deflate” quite easily, while those below it could provide progressively more resistance. It would likely need to be much thicker than the typical stuntman air pillow, but I think that would provide the best, most repeatedly successful results.
