I think some escaped by boat - but I’m just remembering that from a movie or documentary.
Some were probably out in boats to start with - and there are always people who can ‘see things coming’ and may have helped others to get out. I’m not real familiar with the history of the eruption and what records have come down to us.
We know a lot of the details about that day because of Pliny the Younger whose historical letters describe his experience during the eruption while he was staying in the home of his Uncle, Pliny the Elder in a nearby town.
There is an excellent best seller novel that covers much of the detail you are asking about. “Pompeii” by Robert Harris. You can read about the book here: