Surely it's only water damage if you ... damaged it with water? You don't mention going swimming with it in your posts. If everything else about it still works, I'm surprised only the hardware buttons are failing?

Anyway, regardless of that sticker, get Nokia to look at it first. I guess that sticker is there for when the whole phone fails to work and a tech can quickly guess what's to blame. If only one thing doesn't work then hopefully that's a different story.

If they won't/can't fix it then someone with good torx screwdrivers will at least be able to clean and re-seat the hardware button ribbon connector quite easily.