This guide will show you how to go past the sign for the competition arenas in Lion's Arch and go into the water. I have seen many people constantly trying and trying to get through the just barely obtainable distance between the cement pillar and the black rocks, but this guide will show you how to get down there on the first try, every try.
First i will establish exactly where this is for anyone who isn't familiar with the glitch.
If you go to the arena guard for competition arenas in Lion's Arch and Hold down Alt, You will see in yellow letters "Competition Arenas" on a sign. This sign is the key to reaching the water.
This is a view of the general area of the glitch.
Ok now that you're in the area, go over to the stone pillar "competition arenas" sign and stand directly next to it, facing out toward the sea. Hold down forward and make sure that you are as close to the water as possible. You will be just a bit farther onto shore than the sign.
Now that you are facing the sea, and next to the sign, just hold down S or DOWN and don't do ANYTHING else. You have to keep moving back untill you pretty much disappear from sight under a hut.
Here is what it should look like from above view and from front view:
Once you have backed up as far as you can, zoom out your view all the way and look at the competition arenas sign.
Now move your mouse onto the competition arenas sign (WITHOUT CLICKING) and move it to the left where a small bit of sand touches the water.
Using the picture above, put the mouse exactly where that circle is and click. Watch the magic! aka some guy working late at arenanet who didn't care about details.
Well that's my guide on how to get into the dock water area in Lion's Arch.
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Note: If the picture depicting exactly where to place your mouse is not large enough for you here is a larger version.