It took 9 hours to rescue the 500,000 people trapped in Manhattan that day. It was the largest sea evacuation in history, larger than the 339,000 British and French soldiers rescued at Dunkirk in 1940.
Some might say that the 9/11 boatlift was not as great perhaps because the Allies at Dunkirk were encircled by the German Army, knew they were in a war, and were rescued over a period of 9 days in 900 vessels which were much slower than those used 61 years later. Yet the reality is that on 9/11 no one knew what was happening, and as one person explains in the video they thought that the boats in the waters of New York City might also be a target.
Anyone who remembers that day remembers that everyone thought that almost anything was possible – and that their town, their place of work, the nearest power plant, whatever, might be the next target. Hence, it is not surprising that people were eager to leave the site and that the waterways were considered almost as dangerous.