The towns around the Bay of Naples were also the locations of the country homes of Rome's rich elite, so they weren't there a good part of the year. There had been an earthquake in 62 AD that caused quite a lot of damage, and of the homes in Pompeii that have been excavated, a good many seem to have been under repair. Some of the public buildings were being rebuilt, but the work stopped and never resumed after the eruption in 79 AD. Some homeowners (presumably -- perhaps they were ne'er-do-wells) returned after the disaster, figured out where their former residences lay buried, and dug "robber's holes" down into the structures in search of various presumably precious items. In one of the gymnasiums the players of some Roman sport had got ready for their game, had the game balls piled up as needed, then had to flee when the mountain went up.
Your reference to water engineering makes me ask if you have read Pompeii by Richard Harris.
This is a great work of fiction whose hero is the water engineer of Pompeii, a position of some importance. When the fountains and acqueduct run dry a few days before the eruption, he enters upon a search for the cause. it’s kinda an historical engineering mystery based on facts. You will learn a LOT about Roman water engineering (which sounds dry, right) but turns out to be fascinating. First couple of chapters may be slow to set the scene, but well worth the read.