NYPD claiming not terror related:
ABC News: Authorities are questioning three men after pulling them from a submersible vessel in the East River in Brooklyn, N.Y., according to reports from ABC affiliate WABC.
The men were captured near the Brooklyn Cruise Terminal in downtown Brooklyn, near the massive cruise ship the Queen Mary 2, just after 11 a.m.
Apparently the folks in this vessel we're recreating the "Turtle" sub, which sailed near Brooklyn, and became famous on September 6, 1776 during the Revolutionary War.
Update: Thanks to a tip from Publius, it seems that this could be a stunt from artist Duke Riley. Flickr photos.
Newsday confirms all of this and boy are these guys in trouble.