See.... you left out the key bit of information, so the advice wasn't complete. Yes your cruise ship may dock at 7, but by the time you get off, so will 3,000 other people. There's now a bus that goes directly from the cruise docks to the Acropolis area,... but all those other people will be taking it too! And there will be a taxi line -- but again, thousands of people will be seeking a taxi! Your best bet to compete with your fellow-cruiser would be, I'm thinking, to buy tickets in advance for the Acropolis (no there is not a separate ticket for the Parthenon! if you look at pictures, it's sitting ON TOP of the Acropolis), and also book a taxi/car in advance.
There's one called Welcome Pickups, another called CityCab, (google them for e-mail & details) both get good reviews on travel forums for fair pricing & drivers speaking good english. That way, when the cruise throngs pour off the gangplank, lining up at the bus stop and/or the long taxi line, you'll see a driver waving a sign with your name on it. It takes about 30 minutes - more if there's a lot of traffic -- to get from cruise dock to drop-off point near Acropolis entry. If I were you, I'd also make arrangement in advance, for a pickup about 90 minutes before your cruise departs. A good place for this pickup may be beside the AKROPOLIS Metro station which is just about 100 yards from the Acropolis Museum entrance (You can also supply your mobile # for the driver to alert you that he's at the rendezvous point).
With this arrangement, your sightseeing schedule might go something like this: into taxi by 7:30 -- at Acropolis gate if lucky by 8:15 or so (advance tickets or print-out in hand) ... about 2+hours to get up, explore, get down & out the SOUTH entrance.... from there it's just 50 yards or so to Acropolis Museum entry. You can easily spend 2-3 hours there and PLUS, it has a lovely terrace restaurant looking right up at Parthenon, with VERY fair prices. THen depending on your cruise-ship departure times, you could stroll down the promenade to Hadrian's Arch (you'll see it from Museum entry), and Temple of Zeus, you can glimpse thru the fence. A lovely dessert treat along that promenade just before then is FRESKO, a frozen Greek Yogurt shop YUM. THere are also cafes for a drink or a snack in that stretch of shops. If you have a super-long time to sightsee, just about 100 yards WEST of the Acrop. Museum is the unusual (gorgeous Mansion) LaLounis Jewelry Museum... he was the MOST famous Jewelry Designer in GReece -- sort of a combo of Cartier/Tiffany in reputation. And yes they DO have a gift shop!!
BTW, unless your party has a studious member who's done a lot of prep for your journey, you're in luck, if you have a mobile or tablet to download -- R STEVES website has a FREE audio download for a step-by-step explanation of Acropolis ... also of the Ancient Agora, if you opt for the mutliple-site ticket (tho with your limited time, i wonder). Go to the part of the sight that says watch read listen.