Strangely, the game will ask you to head back to Culvert Mansion at this point, so you can report your initial findings to Desmond. Leave the Cathedral and, once back outside, teleport instantly to the mansion. Head on inside, and you'll find Desmond walking around the main floor. However, when you speak with him, he'll be curt, letting you know that he wants you to see this through to the end, not report every little detail to him. Why the game makes you come here to do this is beyond us, but once you've spoken to him, you can then continue on with the actual mission at hand.
The location your map will point you towards is a yet-unexplored location at the northeast end of Point Lookout called the Wrecked Seatub. Getting there is easy enough. From Culvert Mansion, teleport back northward to the Cathedral, and then begin walking in a northeasterly fashion. This will make you run into a location you may not have run into yet, a graveyard known as the Ark
