Inspired by a thread not too long ago about Blackbeard attacking a cruise ship.
Are modern merchant vessels equipped with weapons? Are the crews typically armed? If so, with anti-ship or anti-personnel weapons?
I assume that any ship that may be attacked would be attacked by pirates in small boats and even then only in certain regions (i.e. not in the middle of the Atlantic).
Modern piracy is far more widespread than you may think. However, I am not sure if there is any standard for crews to be armed. They may figure that the pirates are more likely to let the crew live if they do not resist. Whole crews being murdered is not unheard of.
I imagine that most big merchant vessels and cruise ships have an arms locker of some sort.
I eagerly await the answer.
I remember reading an article about piracy in a straight near Indonesia, and it mentioned that the crews use fire hoses for defense, the stream is strong enough to strip paint off the ship and sends the pirates flying, usually overboard.
Can a person be arrested for carrying a concealed firehose?
I hope not 
Ding We have a winner.
Ship’s crews are trained to use many methods of repelling attacks, beginning with foiling attempts to board the vessel and escalating to apprehending the attacker(s). Using firehoses to repel hostile boarders is one of the methods recommended by the IMO (International Maritime Organization).
However, the possession of firearms by crewmembers is “strongly discouraged”, as detailed in the Maritime Safety Committee circular MSC/Circ.623/Rev.3:
From (PDF file): http://www.amsa.gov.au/Shipping_Safety/Marine_Notices/2002/documents/piracy.pdf
“Discouraged” and “prohibited” are two entirely different words, however, yet from my experiences in the field I would surmise that most (if not all) shipping companies most likely forbid crewmembers - even deck officers - from having weapons on board their vessels.
The US Military is tacitly involved in suppressing piracy in areas like Indonesia where billions of dollars of shipments are seized and fund organizations like Jamal al-Islamiya. Yes, billions with a B. In international waters, you may carry whatever weaponry you like. Piracy is a big problem, but don’t count on being able to carry that Glock 21 into a new york port just because you’re a first mate on an oil tanker.
Additionally, mounted deck guns would be illegal.
I have to admit that one of the more comforting thoughts I had while aboard the two Navy ships I was stationed on was that we wouldn’t have to deal with pirates.
-Tofer
Apologies for the hijack (though it seems the OP has been well answered already) but I just wanted to ask, is the water used in sprinkler systems and fire hoses onboard ships taken directly from the ocean? It would seem the logical thing to do but logic doesn’t always prevail in my experience 
Would that be just in most (or I expect all) countries’ territorial waters, or is there some international agreement about mounted deck guns in international waters?
In every situation I know of, you are correct, it is taken from the ocean. It typically has a static or moving screen at the intake to keep fish/large debris out of the pumps.
Regarding weapons: two relatives of mine owned three large fishing boats (more than 100’) off of Alaska, and due to the risk of piracy (which did happen all the time), they carried fully automatic and semi-automatic weapons on board, in addition to shotguns, hunting rifles, and handguns.
They tell (although I have no proof) of one time where a ship ignored their hails and came too close, clearly staffed with a scruffy-looking crew that was armed with handguns and other non-firearm weapons in clear display, and then someone on my relative’s boat fired a fully auto burst from his Uzi into the air. The boat turned around at once and never returned. Pirates respect some things, apparently.
From the link given up higher:
I find this a painfully naive statement that will cost innocent lives, but that’s IMHO, not fact.
Who cares for a firehose that ***can * ** be concealed?