"...at a force of 0.2 megapascals. That figure might not mean much to most people,"
First, megapascals is a unit of pressure, not force. Second, it would have taken a few seconds to convert 0.2 MPA to 29 psi for American readers.
That said, it's a decent idea. Although I'm rather partial to flamethrowers armored with steel plate pointing down the gunwales. Guess I'm just a little bloodthirsty when dealing with pirates.

Interesting idea. Dangling a whole heap of high pressure water hoses over the side.... and running them. And despite the claim that all that thrashing around and the design of the unit will not damage the paint - unless the hoses are made of Teflon - they will rub and ding and abrade the paint off the side of the ship... As to why people who own the ships, not simply arming the crew and a watch kept is beyond me. A sweeping camera or two in the crows nest - with IR capability for night time use. Half a dozen rifles with automatic fire and night scopes, a bit of thick steel plating at the bridge ends. Problem solved.

I would use this as a backup or last resort, not as the only means against pirates.
Why the reluctance to use deadly force?

Mr Stiffy...
They are ship crews, not military personal. Excluding some countries (or states to be more specific) not everybody wants to fire a gun. Good luck finding a crew who is trained in combat and can reliably do their task on the ship as well.
And unless they painted the ship in facepaint I think it has a fairly reasonable chance of withstanding a couple weeks of fire-hose 'abrasion'.

Instead of ammunition for said sentry guns why not combine with this and make an automated watergun? Hell I will build one if anyone is interested in buying. {[Patent Pending]}

Wow....really? So....what happens when the pirates get mad and instead shooting a few stray rifle rounds in the ship, they start lobbing a few dozen RPG-7 rounds and the ship sinks?
Only a matter of time before the pirates figure our how to catch the hose and use is as a rope.

This has to be one of the stupidest things I've seen yet on Gizmag. Is it April 1st again? I agree with Mr Stiffy (can't believe I just said that either)!

Mr Stiffy, because there's this idea among many countries that people don't have the right to defend themselves. Try to enter a port with an armed crew and they'll be arrested.

Can't blame port authorities for not liking it when armed foreigners show up, can you?
Anyway, they'd never win a gunfight with pirates. A cargo ship is a really, really big target, and these pirates have RPGs designed to take out tanks.

@VoiceofReason
The bridge on most ships is usually 5 to 8 stories above the water = Height Advantage.
The ship is BIG and stable = aiming and accuracy advantage. , The pirates have small boats bobbing around in the ocean = total disadvantage.
The steel plate composing the ends of the Bridge provide full body coverage against most gun fire. The Pirates are getting shot at from above in open boats.
In terms of who can shoot who, a few hundred rounds down into the smaller boats, from a great range, with very remote chance of getting hit ones's self - it's a great idea.
Until the pirates get self guided man portable missiles.... but that is another issue of COST and when they revert to this, instead of simply raiding the ships and taking hostages, then they risk the comeback of openly being attacked themselves with more and better armed crews on the ships... including military coverage and hardware on board.
2 or 4 soldiers onboard - with extra hardware.... A few hundred rounds through a 1/2" caliber machine gun....
Personally I have never liked the idea of bending over with my pants around my ankles for any one.
Pirates, thieves and stand over merchants even less so.