A sea kayak, and go where?...

Martin is about right. If you were to bug out of Manhattan afoot, well, the movie "Warriors" comes to mind. I hope you are well armed if your gonna hoof it back through Jersey City, Seacaucus, and Newark. Going into Brooklyn, Queens, or the Bronx by foot via outbound Manhattan routes won't be any more pleasant. If I were in Manhattan when the bubble popped, I would be inclined to stay unless:

A: I could commandeer one of the helicopters at the south end

B: I could comandeer one of the yachts and make my way upstate via the Hudson past the Tappan Zee

C: The radioactive fallout is such that I probably stand a better chance getting off the island and facing the hoodlums than hanging out at the local pizza joint.

Taking a kayak out along the Hudson or the East or Harlem rivers will most likely result in you getting run over by one of the above mentioned yachts bugging out. Going out via Southport you will have to be mindful of the jumpers trying to get off the Brooklyn Bridge to evade the marauding hordes streaming in from Brooklyn.