I always wondered what the porthole item was. It is found on the docks behind Anna's hound in Arnika. I just figured it was some random item in the game.I happen to have a Gadgeteer in my party, and I'm working my way through Marten's Bluff. On a whim, I tried merging the porthole with the "pot with hinge" found next to the trap door, and voila! I now have a Port-O-Potty, which delivers Noxious Fumes. It requires Engineering 20 and Gadgeteer Level 5.
It isn't in the official Strategies and Secrets guide book, and I never knew that existed, so I'm passing it along.
Thanks,
Pischle