Weapons are problematic, lots of ports won't allow them in, most crews aren't chosen from merchant marines academies. Many foreign sailors are on the boats because of criminal problems, they are a rough bunch. As a captain I would be very leery of arming a foreign crew. Walk around Dutch Harbor, AK on a Friday night and tell me you want this bunch to have automatic weapons.
Lastly, without weapons training, trying to hit anything on a bobbing boat is hard. Weapons are hard to maintain in a salt water environment.
I’ve maintained weapons in a salt water environment for decades, and hitting a skiff full of pirates is certainly easier than shooting down a flying duck.
A fire hose is good for boats right along side out to maybe one hundred feet. What do you do about the boat with the RPG gunner when they stand off at several hundred yards and offer to poke a hole in you at the water line?.
Your arguments against arming a "pick-up" crew of assorted nasties is well taken. Small arms are problematical in effectiveness for people insufficient training. So instead of arming the sailors, why not arm the ship?
I would propose to place one or two ISO standard shipping containers with a roll up door similar to those corrugated fire doors in the seaward side of the container, just above deck level on both sides of the deck. I would load each container with a 25 or 30mm chaingun. These automatic cannon can fire single shots, low rate auto fire at 100 rpm, and 200 rpm at high rate. They allow you to switch from high explosive to incendiary rounds with the flip of a switch. The best part is that they are capable of remote control from the bridge thus keeping the crew away from mischief.
The solution I propose would add one or possibly two additional bridge personnel as ordnance officers and maybe not even that if you could use the port & starboard lookouts as remote gunners. The containers would be modular allowing you to swap out your ordnance with minimum fuss and bother for maintenance and repair and perhaps even reload of the container mounted magazines.
Regards,
GtG