The notice to mariners, which was first reported by Reuters, follows U.S. President Donald Trump’s threat last month to fire on any Iranian ships that harass Navy vessels.
“Armed vessels approaching within 100 meters of a U.S. naval vessel may be interpreted as a threat,” according to the text of the notice…
So harassment with unarmed boats (which may or may not conceal explosives, mines, limpet mines or rudder/screw fouling gear) is okay I guess… 
Heed the warning. Not so hard to understand stay the fuck away. Our sailors mean business and protect themselves. They finally have a green light to do so.
The warning doesn’t say visibly armed. Just armed. Subject to interpretation and verification after the fact, if necessary, one suspects.
Better perhaps to not be visible, but well armed. OK with that, the warning was put out. You won’t see my weapons until you threaten me. Then we do a dance.
Better to be locked and loaded, just to be on the safe side.
Yes. Better to have it and not need it than need it and not have it. But… I always have my Mutt with me. My black bear can eat the ass out of an alligator.