In the Adventure "Dungeon of the Mad Mage", Skullport is connected to the dungeon of Undermountain via various tunnels and the river Sargauth. However, the adventure does not seem to indicate any distance between the two. While I can try and triangulate an approximate distance, is there any explictit information on this, maybe from older game materials / editions or from official fiction?
About 500 feet
The map in Dungeon of the Mad Mage is almost a direct copy of the mid-East portion of the South section of the Sargauth level map from the AD&D 2e boxed set The Ruins of Undermountain.
The expanded map from AD&D 2e shows Skullport immediately down river in a location called "Skull Pool" from where the Dungeon of the Mad Mage map cuts off. The grid matches up between the old map and the new one, so I used the new scale of 10 feet per square. This gives us a figure of 500 ft.
Does this match textual references?
I could not find any textual references to the distance from the Sargauth level of Undermountain to Skullport. I checked all of the original sourcebooks I could find that reference Skullport including:
- City of Splendors: Waterdeep
- The Ruins of Undermountain
...and the closest I could find was references to the route, but no specific distance:
The river [Sargauth] flows with a moderate pace along Level Three of Undermountain; it flows from north to south, the waters running down to Skullport.
Depends on how you want to measure it
(but you could very well say that "0 ft")
I pulled up my copy of the Forgotten Realms Interactive Atlas (this is 2e material, but I doubt canon has changed much) and stitched together (somewhat poorly) the North and South parts of Undermountain's Sargauth Level. This is what it looks like:
I added a couple of vector measurements for reference, but really... Skullport can be said to be inside Undermountain.
In case it's not clear, the purple rectangle in the bottom-left is Skullport. Note that River Sargauth reaches Skull Island.