Keep it wet if you can; covering it with wet newspaper helps.
It will take a long time before live rock will dry out and die. Think of how it is shipped in the first place -- from wherever it is harvested to a wholesaler. That probably takes several days. Then from a wholesaler to your LFS or an online retailer -- that's a couple days in transit there. It's not kept submerged in water that whole time -- it would be too expensive to transport all the rock in water. Then it goes from your LFS or an online retailer to your house -- most online retailers will send it overnight, wrapped in wet newspaper. It will be perfectly fine for the car ride home.