Portal 6 is Nyzul Isle. This portal is actually inside the Alzadaal Undersea Ruins, though not in the restricted areas used in Salvage. The best place to start is the same entrance you use to complete the Aht Urgan Mission.
Take a silver piece up to the northwest corner of Bhaflau and pay the toll to get in. You'll see there're platforms in several places throughout the ruins. Blue portals lead one way, white portals lead the opposite way. Use the portal in that first room to be taken to the main map (bought for 3 myth pieces, I believe). Head north or south to the next room, but don't step on the portal. Instead, head as far east as you go. Whenever you come to a fork in the road or a room, take the east path or the east exit. You shouldn't be in danger of any enemies; the only ones I've seen were qiqirn, and they didn't aggro me. Then again, I haven't been through there a single time without seeing someone solo there, so they may have cleared a path for me. Doubt it, though.
Keep heading east and you'll get to the staging point. You'll see there's no Immortal guard (currently) but there're two of those tiki-torch teleporters. Click either one to go whizzing back to Whitegate.