I recall some systems (linux specially) having issues with texturing there, showing black areas in front of the citadel.it was not near the waterfall, it's right near the entrance. i'm a mac user so it might be mac specific. I can walk on it, I fell down a regular cliff that I couldn't see because it was black.
Although, that meant (I think) non lighted textured (cached lighting not found) but I guess you could walk on them, normally.
Seems that, what you found, is an actual hole in the map. If it isn't the hole where the water falls... :P Try approaching that spot once more, and use the /pos command so the exact location can be stated and someone might take a look there.
-6.34742 -271.245 -2599.74
0.983483 -0 0.181003
-0.00598293 0.999454 0.0325084
-0.180904 -0.0330544 0.982945
"bdoorsout"
0