1. Liz bought something at a grocery store. Liz says to the cashier, "A box of chocolates and two colas."
This is both the box and the chocolates inside the box.
2. Liz's mother Pat bought Liz chocolate. Liz says to herself, "Wow! How big! I can't eat a box of chocolates at a time."
This is the quantity of chocolates that a box of chocolates would hold. She will not eat the box itself. It could also be said, "I can't eat a box worth of chocolates at a time" or "I can't eat a boxful of chocolates at a time."
3. Chris and his son Jim are washing their car. Chris thought he needed some water and a bucket. He says to Jim, "Go fetch a bucket of water."
This means the bucket and the water in it.
4. There was a big earthquake and suspension of water supply happend. Chris and his son Jim are measuring water to deliver to the people of their group. Chris says to Jim, "Water is short. We need one more bucket of water."
This means the quantity of water that one bucket would hold. It could also be said, "We need one more bucketful of water."