Spider-Man: Homecoming - Predicting The Plot In 8 Steps
5. Peter Attempts To Stop Vulture Transporting Weapons
And that decision will be to attempt to interfere with Vulture's weapons operation.
That operation will involve the transportation/smuggling of said weapons; Vulture could be selling them on, or moving them to a safe location for later use. He's using the ferries to transport his goods, and once Peter discovers this, he chooses to strike.
Peter will directly confront some of the Vulture's goons - perhaps they've assumed control of the ferry, which explains the lack of security personnel - at the start of the Staten Island ferry scene glimpsed in the latest trailer. This is the moment he jokingly mentions their "weapons barrier".
But, as we know, it all goes horribly wrong.
Vulture doesn't like the fact that Spidey is trying to mess things up for him, so he attacks the webslinger, leading to the boat splitting in half.
Iron Man then arrives and saves the day, but it's not all sunshine and rainbows - Peter's actions almost got a bunch of innocent civilians killed, and Tony recognizes this fact, taking the youngster to one side for a well-earned scolding.