I hope this wasn't posted before, Ifound this yesterday more or less by accident and I think it's a funny thing to share :S. I thought it was Mage only because I did it with the Blink exploit at the gate (which can be done with auto-unstuck) and after that working with Blink and Slowfall. But today while PvPing I found another way for every class. I'll explain after some screenshots.
It's not really useful. And you can get there simple by getting under SW (I will not explain that :S) to the Spot where I am standing in this screen:
If you have an epic mount just simple ride forward towards the water and you'll start swimming when you reach it. If you swim out of the water box you will fall down and die (DOH!). Didn't test it with normal Mount, but i guess the water box is deep enough (there IS a bottom). Oh and btw, you will just see out of the water if you are near the border of the box and zoom of it with the camera. Else you will just see endless water :S
I have found absolutely no use for this, since the water box has a bottom and is just as big as the pond itself. Just a nice little exploit.