What other purposes would a small naval vessel firing an EMP blast at the US coastline have?
A small naval vessel can’t carry a missile big enough. Something like a Standard Missile doesn’t fly high enough. You need an IRBM at least, and there aren’t many ships that can carry them.
I won’t get into details, but there aren’t many merchant vessels that can carry them, and we know where they are all. If you’re idling off the coast to erect a missile, you will be found.