This is simaler to Korben and Neons way of getting into "grey hell"
First of, buy Prostitution business next to your safe house in Vice Point (The Clymenus Suite)
The get a PCJ and take a long run at the building (I suggest from Ocean Beach bridge, just near the construction of the mall). Then go in at the building at the left hand side and stay in the far left corner of the building until it loads and then you should drop into the water underneath the city.
Picture: [attachmentid=6049] [attachmentid=6050]
Thanks to Korben and Neon for finding this original find of going through buildings