The term FIRST HOUSEBOAT is composed of two words, the FIRST being 'FIRST' and the second being 'HOUSEBOAT'. The crossword solution for the FIRST word is 'ARK', which is a three letter word. An ARK is a large boat or ship that was used in ancient times to transport people and goods across bodies of water. The second word, 'HOUSEBOAT', refers to a boat that has been modified to be used as a residence. The combination of these two words, 'FIRST HOUSEBOAT', refers to the FIRST boat ever used as a residence.