Having been certificating airliner galleys until recently (not Boeing though), I can hopefully shed a bit more light on the plumbing. It is designed to operate at 40psi, but is designed to take 125psi when being refilled and is static tested to 188psi. Ultimate burst pressure is supposedly 250psi, but they will take more...
. All potable water hoses were PTFE lined and reinforced with stainless braiding, whilst waste hoses were reinforced silicone. Rigid pipes and unions were stainless steel.
The water heaters I used were four litres capacity, with a stainless steel reservoir. They have pressure relief valves and generally don't heat water above 88°C on electrical power due to the drop in water boiling temperature at 8000ft cabin altitude.
A B777 size aircraft could have easily have six or more galleys with plumbing for hot water boilers, beverage makers, coffee makers, espresso/cappuccino makers and steam ovens (Google 'airliner galley inserts').
If you need to get a higher pressure, wrap the boiler with steel wire. Look up 'flash steam' for high power steam systems with small boilers.
If you can get them out, the engines and airframe will contain thousands of bolts and studs from which tools and weapons could be made. Also, on the subject of weapons, if you have a hydraulic cylinder, some jet fuel and oxygen (bottled or generated) plus a means of making a spark, you can make one hell of a cannon!
With all the aerofoils or curved surfaces that are available (slats, flaps, control surfaces & bits of interior trim) why not build a wind turbine? You could make a peristaltic pump or motor out of a wheel and an inner tube (there must be a bicycle in most airliner holds) as some Scottish crofters did on a TV programme (The Great Egg Race).
BBC - Archive - The Great Egg Race - Cracking feats of engineering
A visit to an aircraft breakers would be a good idea to get an idea of what is available. Googling B777 suppliers will yield photos giving an idea of what is in its sub-assemblies.