I've made two trips to the Channel Islands, using large ferries that sail between Normandy and southern England. Of five planned voyages, two did not take place because the ferries were canceled--one of them for three days in a row. If I had a tight schedule, I would not depend on one of those ferries. If something goes wrong, you'll really be scrambling, and you probably won't be near a commercial airport.
For seeing the invasion sites, I highly recommend one of the small-group tours or a private guide. I was very pleased with my Overlord tour. With a professiomal driver/guide you will move efficiently from site to site (they cover a wide area) and you'll get lots of helpful information. You will know what you are seeing. If you try the do-it-yourself approach, you may feel like you're looking at beaches and chunks of concrete. That would have been my experience, and I'd have been bored after an hour unless I stuck to the museums (which I love, but I think most folks go to the beaches, which without a guide's explanation look like pretty much any other beach).
Most tours depart from Bayeux, which has the Bayeux Tapestry, an important cathedral, a very good invasion museum, and an attractive historic center that didn't get obliterated during the invasion. So Bayeux is usually the best base for people exploring invasion sites. Other good museums are the huge Peace Museum in Caen and the Airborne Maueum in St-Mere-Eglise. The Airborne Museum was included on my tour. All three museums have their purpose, but I'd want to see at least one of the more comprehensive museums (in Bayeux/Caen) if I had the time to do so.