To get to the deck of the Chicago Overcoat, you must either fly there from the surface of the Imaginary City Streets or one of the four kingdoms, or ascend the spiral staircase in the back room of Mobster Kingpin's office. The window in MK's office door acts as a portal stuck on the stern of the ship.
The Chicago Overcoat is where Team Sleuth has chosen to make its final stand against DMK. When DMK first appeared, he destroyed the masts of the ship and caused the fans that were propelling it to scatter.
Andrew Hussie named it after the mobster slang for "coffin" - namely, the cement that you encased an undesirable character in before dropping them into a body of water. This was then referenced by Lrd English's CAIRO OVERCOAT, which itself had a sarcophagus - an Egyptian coffin - as an innocuous double.