I think many cargo ships of various kinds do routinely carry weapons. I don't know about the big company supertankers and such, but it wouldn't surprise me at all.
The best weapon they have though, is the ship itself, and maneuvering. A three or four hundred foot tanker moving at full ahead is not something anybody will wanna come very close to, especially if they are trying to not be boarded.
Boarding a ship that big moving at speed is not a trivial problem. It's complex enough when the crew is trying to help you board, but if they are in a "repel boarders" mode... its gonna be tough to pull off. Somebody pulls close aboard and tosses a grapple up... they're pretty much going to die if they think they can climb that rope.
I have heard of them getting successfully aboard freighters, but agree it would be damned difficult under the circumstances you describe. I always keep a good distance from freighters and tankers off Miami, anyway it makes the skippers nervous if you come close.