One diamond
The very first reason I would say is because it doesn’t specify how big of a diamond meaning the value could range from $100 to $100,000,000
The second reason is water is free in a sense you could get five empty bottles you find laying around and then go to a creek and fill them up with water. (You could get so much more for a diamond than a few bottles of water)
The last reason ( that goes along with the first two) is because it’s unique diamonds are hard to come across but water in the other hand is found almost every wear in fact it covers about 71% of earth.