Water-powered jet pack lets you swim like a dolphin
kottke.org
kottke.org
4900e sounds very affordable, but I suppose you need a jetski to power it
"Fly Board: Acceleration from a stand-up PWC (aka jet ski), 4900 euros.
Optional: Acceleration from the Flyboard directly, 900 euros."
If you watch the video again, you'll notice that Flyboard isn't attached to a PWC but to something smaller. There's an another video [1], where you can see a close-up of it on 04:27.
Saw this a few months back — totally cool. I'd imagine that you'll start seeing them in your highest concentration touristy beaches for rent soon.