I think that if Brook was demolished, it would be like when Barry the Chopper was torn up in Fullmetal Alchemist. Just like Barry was a soul possessing a heap of scrap metal, Brook would be a soul possessing a bunch of bone fragments. Also, Luffy still being rubber when underwater in the Arlong arc was a bit of an exception. His body's structure has been permanently changed, he just can't use it. He can't stretch, but he can be stretched. Brook, I think, is being tethered to life by his Devil Fruit, and the water would break that anchor. However, once he was out of the water, his soul could repossess his body and he'd come back to life.
As to how long he will live... I wonder if the Yomi Yomi no Mi (I need a good English name for that fruit) can be used multiple times. As in, if Brook is immortal. He did say in chapter 488 that he'd never died before, implying that it can be used multiple times. But unless someone kills him again, I don't think we'll find out. I do remember him saying something about coming to the end of his second life, though. And since Brook came back to life with a skeleton, instead of a full body, I don't think he can be killed, because his organs won't wear out or anything, because he doesn't have any. And it seems that his bones will last forever if he has enough milk. Unless the Yomi Yomi no Mi only provides you with another lifespan equal to your old one (your natural lifespan, not how long you lived), in which case Brook will die again, say, 90 years after he came back to life (it's been 47 so far). If it just brings you back to life permanently, and you won't die again except through old age, illness, a wound, or something like that, then it would seem that Brook is immortal, since those things can't affect a skeleton.
)? Where does the food go anyway?
