Tickets can be purchased at this moored station, in front of the Hermitage. It's very easy to find and boats run every 30 minutes from 10 a.m. When we asked what time the last boat would be returning, we were given a vague "5 maybe, 5:30 maybe" answer, which meant we had to cut short the visit in order to ensure that we weren't stranded. This mode of transportation leaves you a bit short on time to actually explore and enjoy the palace and grounds.
The cost? 600 rubles (20 USD) per person, each way. . .draw your own conclusion. :-)
The Hydrofoil ride to Peterhof (29 kilometres from St. Petersburg) takes 45 minutes, crossing the Gulf of Finland. Although most of the write-ups I've read tout the scenery and beauty of this approach, I wasn't really impressed by it. However, if you're on your own, it may still be the simplest way to get there (depending on season ~ it runs only when it's warm enough).
Probably the best way to visit Peterhof is to take a hydrofoil across the Gulf of Finland to the jetty at the end of the Marine Canal. Three companies run regular hydrofoil services through the summer months from the Hermitage jetty, Angliskaya Naberezhnaya next to the Senate Building, and Universitetskaya Naberezhnaya next to the Academy of Sciences. We took the one from the Hermitage. We napped a little on the ride because we had gotten up so early, and I also took photos of some of the aids to navigation that we saw along the way.
We hired a guide for one of the 3 days spent in St. Petersberg. It's not the cheapest way to go but if you have limited time, it may be the best option. This was pre-arranged before leaving the states and included our guide, a van, driver and a custom tour. It was definitely our best day and we never could have seen so many sights on our own.
We visited the Catherine Palace, Pavlovsk, & Peterhof plus a few churches, and rode the metro. Since we were a small group of 3, our guide got us past the very long lines in the palaces. We had a very full schedule and didn't even bother to stop for lunch!
Our guide Nina is an English teacher during the week and gives tours on weekends. She was fantastic and extremely knowledgable!! Unfortunately I don't have any contact information for her.