Herculaneum is a stop "Ercolano Scavi" on the same train. That all day ticket is like a hop-on, hop off tickets, good for that train, trams and buses--make sure you tell the tobacco guy you want the all day ticket to Pompeii and return also good for the trams--You can also say "biglietti train a Pompeii et return con tram"--and motion to the tram line, in case he doesn't understand English--which I'm pretty sure he will, as his main market is English speaking tourists.
Once you get off the train at Ercolano Scavi, it is a short downhill walk, uphill back--not far as the train station at WDW to the hotdog place at the end of Main street. The entrance area has A/C and good bathrooms (LOL). You have to check you backpack there (very safe). H is not crowded and much smaller and more shade. The buildings are more intact and you can still see some frescoed walls, etc. It is a good add-on to Pompeii (you could spend a total of 2 hrs--walking, looking around, back to train) and get a decent look around---but Pompeii should still be your 1st stop and the earlier, the better (definitely off the boat by 9am--8:00/8:30 is ideal).
I think Pompeii opens at 10. There is also a tour bus that goes to Pompeii and back--the stand is right at the end of the port--before you cross the small street to the tobacco stand. We debated taking the bus--but it returns at a set time and there is no option to go to H then. We opted for the tram/train. If you have to wait for the next train--we had to wait 20 minutes, there is a fun little food/drink stand right by where you get the train. My kids got some 1 euro interesting fruit slushis (like coconut, etc.) while waiting.
If you surf the web, you can find step by step photos of what to do to get the tram/train, etc. My info comes from those, which I followed in 2011--only getting off at the wrong train stop on the way home--but easily getting on the train back 1 stop. Aug. 2013, it was a breeze. We felt very safe both times--of course, watch for pickpockets--but I carry a shoulder bag purse and never had any issues.
As I said, if you can get the little book for 12 euros or so, there is no need for a tour guide--just get a map with your ticket--either free or 1-2 euros extra. They also had that Pompeii book a at Pisa and probably places in Rome, so you might look for it at earlier stops. We have seen many tour books--but the overlay with cardboard is excellent--for adults, as well. If you want a tour guide, they are 10 eruos PP at the front gate. The book is 12euros--and I was the guide. I also really like DKI European Ports--I have used it for years--worth buying. have fun! Elaine