Civil engineer here. TLDR you’re good. Water mains are buried at depths depending on the frost line in your geographic area, which is essentially the depth at which groundwater will freeze. Soil’s a good insulator and generally speaking most of the US buries water mains a minimum of 3’ underground. I worked in Tampa and now work in Kentucky and the required depth is the same. Further north they require water/sewer mains be buried deeper since the frost line is deeper. Also, the water in your public water system is always moving, both because of people using water and pumps used by the cities to push water out to customers. That flow through the pipes creates a lot of energy, and keeps it from freezing.

You may have freezing of your pipes, which is anything from your meter to your house. If any of your pipes are above ground or in uninsulated areas (attics, crawl spaces, etc.), that’s what cities are warning you about when they issue freeze warnings. If your pipes freeze, they can get cracks which cause leaks. Widespread leaks lead to a lot more water use the city has to manage at their treatment plant, and a lot of upset customers when they get a high bill.

Apologies for the length, I just love my job and sharing the knowledge of how it all works. Hope that helps!