There was really no threat of invasion though. While the final battle between the Mexica and the Native/Spanish forces is often lumped together as the "Siege of Tenochtitlan" there were skirmishes and battles happening around the Lake Texcoco basin.
I had always imagined that the Siege was mostly an overland affair, never really taking into account the fact that the whole city was on an island. Yeah, sure, the fact Cortes had ships built is mentioned, but you can’t have a clear idea of how they were used with two or three paragraphs in a school history textbook.
It shows the influence of modern geography on the perception of events that happened centuries ago.