This is, of course, not the Isis -which was bound for Piraeus (Athens port city) in the story. It is, however, a Roman grain merchant ship and the mosaic is from Ostia, Romes port city, which is very much comparable to Piraeus when it comes to grain transports.
"what an enormous ship it was! A hundred and twenty ells in length, as the carpenter told us ; upwards of thirty ells in breadth, and from the deck to the bottom of the hold, where the pump stands, nine and twenty [55x14x13 m]. And what an astonishing mast! What a powerful yard it carried! What monstrous ropes that hold it fast on either side! Did you observe, how the stern rising insensibly in a curve, was ornamented at top with a golden goose? And how at the other end, the prow proportionately swelling forwards, had on the two sides the ftgure of the goddess Isis, from whom as its patroness the ship takes its name? To say nothing of the other decorations and appurtenances, the paintings, the purple flags, and especially the anchors and capstans and windlasses, and of the cabins disposed in the afterpart of the ship. All, in one word, all appeared to me admirable. The sailors, in such a surprising number, that they might be compared to an army, and the cargo, it was said, was so large, as to be sufficient provision for all the inhabitants of Attica [ca 300.000 inhabitants] for a whole year."
Lucian - The Ship of Wishes 435-437
Translated by W. Tooke