Matty Blast

Chosen answer: During the several years it took to construct the ship probably, or in any of the supplies/food brought on board, or in the furniture brought on board. A single pregnant female rat can be responsible for thousands of rats in a very short space of time (the offspring are not too choosy about who they breed with).

Soylent Purple

Answer: In addition to the other answers, rats can easily get on ships by climbing the mooring lines that tie vessels to the dock and also go up unattended gangways. They can also use temporary overhead cables attached to ships while in port.

raywest ★

Chosen answer: No real artifacts were used but they did use real film taken of the actual ship in the movie.

Disney-Freak

Chosen answer: This is what really happened - it was at first feared that the davits used to lower the boats would buckle under the weight, or that the boats themselves would break in the middle, and so many of the ones lowered first were not filled to capacity - one of them, with space for forty, had just twelve people on board. Later, as it was realised that there was not nearly enough life-boat space for all the passengers and crew, boats were filled to capacity, or even over-filled (one boat carried seventy).

STP ★