On the Preplan/Structure List Map View, we show Water Supplies only if they are associated to or nearby a preplan/structure.
The reason being, that some orgs have imported 12,000+ hydrants (which can be seen on the Water Supply List Map View)! If we showed all available water supplies, the map view could crowd out your preplan icons...
The Preplan plus Water Supply view could look like this (even though there are 2,000+ hydrants in the pictured organization):
Here is the water supply view for this same org: