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
Either the captain or first mate would be sufficient to determine the armed resistance. But complicated weapons that have to be maintained in a salty environment would really be problematic. The boat owners are not going to give up any cargo space to weapons.
Sure if the pirates want to use RPG's they will, but it's bad for business to disable or destroy a ship, no payday. Piracy is a business.
Fire hoses can discharge 2,000 GPM easily 300 feet and fill one of those skiffs. What about a smoke screen?