We booked the jeep by calling Hertz over the phone about 2 weeks prior to sailing.
It was hard to find the place to pick it up at but after asking a few locals they steered us there! It was a little trailer off the pier (maybe a 10 min walk).
Our ship docked about 7am so we left about 8am. I believe the ferry leaves every hour.
CHeck this website out: http://www.vinow.com
They have alot of St Thomas/St John info on them plus ferry schedules. Our ferry we picked up was the 9am ferry. When you get a taxi (they line up at the dock when you get off so it's very easy to hail one!) tell them you want to go to the Red Hook Ferry. They will know exactly where to take you.
It is alittle ride (maybe 15 minutes to it) and then the ferry is about another 15 min ride.
We probably got over to St John about 9:30 and picked up the Jeep. It was really easy cause we had booked it early.
We picked up a map of St John (You can get one online or at the rental place) and just drove the outer roads (there is only like 2 main roads there anyways. There are stop points along the way to pull over for spectacular views and information on the islands you'll see in the distance. Really pretty! We videotaped alot of it.
We first drove to the other side of the island and toured the Annaberg Sugar Mill ruins. Not much there but very interesting and informative. We then drove to Trunk Bay and stayed there for about an hour or so, then about 1pm we drove to Coral Bay (St John) and ate at Skinny Legs. We almost drove right past it cause it's alittle place but the food was AWESOME! I advise you to eat there if you have time. Then we drove around alittle bit and dropped the Jeep off about 3pm. Caught the 4pm ferry and then a taxi van was waiting for us and he took us directly back to the boat. Our ship didn't leave til 6pm though and we were back by 5pm or sooner. If you are coming back and have to be on board by 4:45 it is probably doable but make sure you keep track of your time and leave an hour early to allow extra time to get back to the ship on a taxi. We even were able to do alittle shopping at the dock (there's a few shops around) and got some ice cream YUM....before we had to be back on board.
If you need/want more info feel free to email me at firstname.lastname@example.org
or I do have yahoo IM (hkolln is my ID) and we can possibly chat if you tell me when you'll be on. I don't keep it open all the time.