webgiant wrote:If the umbrella is strong enough to carry him upwards, then it is strong enough to act as a parachute on the way down.
Upwards it largely just needs to be big enough to catch as much water as possible, while surviving the force of the fountain.
Down-wards, it would need to have a large enough surface to actually slow you down.
The best way to illustrate might be to imagine a man standing on one of those sewer-covers as water suddenly blows out: If the water has sufficient force and the man can keep the cover balanced, he will be pushed upwards.
When the water-stream stops, however, the man-hole cover will do nothing to slow the man's fall, it itself having a higher density to begin with, and the man being able to create more air-resistance by just lying horizontally spread-eagle like a sky-diver.
So, if the umbrella is too small/heavy, it'll do the person no good on the way down.