I remember taking the train from Sorrento to Pompei some years ago.As soon as we arrived the heavens opened and there was water everywhere.It didn`t last long and an hour later you wouldn`t know it had rained.
Pompei is a magical place but one day isn`t enough.we hired a local guide for a few hundred Lira.His knowledge was invaluble. You don`t have to look for one, they approach you.Herculanium is also well worth seeing.IIRC it`s 1 or 2 stops before Pompei.BTW I used ISO 100 no probs.

Cheers Dave