Again, I like your thinking, mate
You're not stupid and doing the research, I hope we can help (don't count on me too much
)
Watertanks (3 of on 777-300) are fibreglass
Wastetanks (2 of) are carbon fibre, so both are good for storage (although wastetanks???) or as floating devices.
The RAT might survive the impact/landing but only if it remains retracted (it deploys from the bottom of the plane).The RAT deploys automatically when both engines stop....so be aware.
Bare in mind the APU needs fuel to work (they use about 200+ kilograms per hour) the fuel pipe runs under the floor so you might want too leave that intact.
The oiltanks, sit on the engines, are made of casted aluminium, probably melts.....
O, and about the fuel; they
dump it (jettison) and
ditch the plane
I probably go too much into details and I keep thinking about a movie i.s.o. a book (hope for you one day they'll make a movie of it), forgive me for that.
And again, my beloved Triple doesn't fail
