So, "Dave" gets an MRI. It's a joke involving the electromagnetic machine and his metal body. Okay, funny, haha... but not only the test would accuse something wrong(his whole body is metallic!), the ship itself would probably be fried by the electromagnetic field. Yet nothing wrong happens and everyone acts as if it all went normally... how?
Who said the body was made of metal? It could be emitting false readings for the MRI to pick up.
"Who said the body was made of metal?", you ask? Well, the fact that Dave was involuntarily pulled toward the MRI machine almost in a cartoon-like fashion while the magnets were warming up, and got stuck to the side of it, should be a fairly big implication to that effect.
Okay, apparently no one else thought of this, and maybe I missed an explanation in the movie, but... why do they need to drain the ocean to get salt? Aren't there massive salt warehouses where they can get their fix without killing the Earth?