The Dejah Thoris was the Fourth Doctor's houseboat. He took Leela and K9 Mark I to Mars to sail on the canals.
It was constructed similar to houseboats made to cruise in the English canals, with the words "I.M. Foreman (London)" stencilled on the cabin.
The houseboat was destroyed by the Last Martian, but the Doctor rebuilt it. (PROSE: Crimson Dawn)