You obviously can't be invincible in minecraft. It is annoying and strange enough that as of 1.3, a boat will last forever, no matter how many times you run into your friend's docks, and random islands. It would be a lot more fun if you could get totally stranded, and have to find your way home. What would the purpose of all of those hazard markers, that costed you so many materials be now? If you took damage from crashes, minecraft would be a little bit more realistic. It would also be great for noob traps... Wouldn't that be nice?