As they neared it, this was seen to be the head of a swimmer.

He was soon picked up, and was found to be a Venetian citizen, named Savadia, who had been captured by the enemy, but had managed to escape, and was swimming towards land to warn his countrymen that the whole Genoese fleet, of forty-seven sail, under Pietro Doria, was close at hand; and that the six ships in the offing were simply a decoy, to tempt the Venetians to come out and give battle.
Giustiniani at once returned to port, and scarcely had he done so, than the whole Genoese fleet made its appearance.

They approached the passage of the Lido; but the respite that had been afforded them had enabled the Venetians to make their preparations, and the Genoese found, to their disappointment, that the channels of the Lido and Malamocco were completely closed up with sunken vessels, palisades, and chains; and they sailed away to seek another entry through which they could strike at Venice.
Had the same precautions, that had proved so effective at the Lido and Malamocco passages, been taken at all the other channels; Venice could have defied all the efforts of Doria's fleet.
The city is situated on a group of small islands, rising in the midst of a shallow basin twenty-five miles long and five wide, and separated from the sea by a long sandbank, formed by the sediment brought down by the rivers Piave and Adige.

Through this sandbank the sea had pierced several channels.

Treporti, the northern of these channels, contained water only for the smallest craft.