Attach a hose to the drain cock in the base of the heater, open the valve and drain into a container. The plumbing of large buildings will contain a lot of water. The building maintenance supervisor would know how to access this water. Waterbed water is not safe to drink due to the toxic anti-algae treatments, but it is a great source for water for flushing toilets. Treated swimming pool water is a judgment call based on what it has been treated with.
Many cities have artesian wells in parks and other public places.
Unless announced otherwise, purify the water. Most houses and buildings have gutters that collect and channel rain to down spouts. When it rains, let the water run for about 10 or 15 minutes, and then catch the runoff from the down spouts in barrels or buckets. Rainwater can be caught on flat roofs by tarps that channel the water into buckets. Rainwater is very pure, but if the roof or gutters are in bad condition or dirty, purify the water before drinking.