Water = Expensive?

The two highest budgeted movies EVER are Titanic and Water World. These movies don't have very much in common besides the fact that they are both take action in a maritime setting. Which begs the question: Why are movies produced with large amounts of water so expensive? Are boats really that hard to come buy? Do the actors get paid per time they get wet? Or is it the water itself? Is water a finite resource, and we didn't know? You'd think that Willy Wonka and the Chocolate factory would have cost more to create than the glamourous Water World in all it's bedraggled glory. Isn't chocolate more expensive than water? Didn't it take longer to clean and dry Agustus Gloop's costume after he fell into the chocolate river, than it took to throw Kate's Dress into the dryer? Your thoughts, please.