A water tower is a medium-risk, low-reward location.
A water tower is generally a large structure that supports an elevated water tank, whose height creates enough pressure to distribute the water through a piped system.
If one could somehow harness the proper equipment to extract water from the tower, they could use it for a very long time, depending on the amount of water that's drawn from the tower. These are also out of reach from any undead creature that tries to scale the tower.
Water towers do not have the space to hold more than a few people. Because these are so noticeable, the occupants of a water tower may have a bandit and zombie problem. Using fire to eradicate these enemies would be ineffective and detrimental, as the smoke could rise up and also affect the people that used them. They also contain little to no cover, so any attack against the tower by human enemies will leave the defenders wide up and easy targets. Zombies are also bound to group near the bottom, making leaving the tower a very difficult affair.