Question: How was the mayor able to drown Mr. Merrimack in the desert?
Answer:The Mayor, along with Bad Bill and his crew, drowned Mr. Merrimack with the water from the jug. That's why his glasses are seen in the jug after the chase seen with the dysfunctional family.
Answer:It's not specifically explained how but it's a clue about the water pipeline that runs to Las Vegas. It has a secret shut-off valve that the mayor has been manipulating to create a drought so he could buy the land cheap.
Answer: The Mayor, along with Bad Bill and his crew, drowned Mr. Merrimack with the water from the jug. That's why his glasses are seen in the jug after the chase seen with the dysfunctional family.