Falmouth is a great town, I lived there for 20 years and my kids still do. There are no scuba shops in Falmouth, the closest is 'The Dive Locker' in Hyannis. (It's about a half hour drive, that time being very dependant on traffic and season. What takes a half hour at 9pm on a February night, might take an hour and a half on a rainy Satuday in July when the tourists are out driving around cause they got rained off the beach.) Mike Walls runs the shop and he's a god guy. They run a charter boat, I believe its a 25' Parker. Nice boat, the skiper is a great guy as is the mate/divemaster. The boat gets a little crowded with six divers and gear. They run to some great wrecks. They will grill you rather extensively about your experience before taking you out, for two very good reasons, currents and fog. The fog can roll in at a moments notice. It gives me the willies to think about taking some gumbie out and having him decide to do a free ascent/decent in Cape Cod fog. He could be 10 feet from the boat and you'll never see him. And if the currents are running he'll be a mile off the boat in a matter of minutes.
The vis can vary tremendously, but it's typical New England fare, five foot is not uncommon and if you see 50 foot you go home and call all your friends and tell them about what a great day them missed. The Outer Cape is a bit better than that but not a whole lot. The water is still cold, I had 41 at the thermocline at 50 feet last weekend but that was on the North Shore. The Cape will be a fair bit warmer, but don't get carried away, you'll still want a 7 mil suit with a good warm hood and thick gloves. Or be smart and go dry. (I like to be wet when I'm lobstering because you tend to work yourself into some weird positions reaching under rocks to wrestle the buggers out. Unfortunately for you that won't be a problem since you won't be able to get a license to lobster. You need to live here to get a license and no boat captain will let you lobster without a license. They aren't going to chance losing their boat to the Department of Marine Fisheries for someting you can buy for $10 at the grocery store.)
There are a couple of shore dives on the Cape. Garbage Beach down in Woods Hole (a part of Falmouth) is a great dive. You can hit pretty close to 90' (if I recall correctly) from shore BUT Woods Hole has the worst currents on the Cape and it is an EXTREMEMLY busy harbor, with big assed ferries coming and going a quarter of a mile away. Diving there without someone who knows their way around is not a terribly smart idea. (It's named Garbage Beach because of the large amount of bottles, etc that have been found there over the years. Divers are the only ones to call it Gargabe Beach BTW, it's not even a beach, its a little bit of shoreline that divers frequent. Like I said, I lived there for 20 years and I never once heard it refered to as that by regular people. Ask anyone in town where Garbage Beach is and they'll probably direct you to NJ. )
Sandwich Town Beach is an interesting dive. It is a clay bank a few hundred feet offshore. You'll need a shovel to find 30 feet. It's cool cause the lobsters dig all these little pockets into the clay and back into them. You'll be swimming along and see their claws sticking out of the hole and they will be gone deep inside when you get over to them. It is a pretty dive, there is a lot of life there, especially at night. It is a local fav with area dive shops for AOW night dives.
The problem with diving the Cape is that it's a big sandbar, with lots and lots of wrecks around it. The wrecks are cool but if you aren't on a wreck you are diving a sandbar. For example, the Port Hunter is a WAY cool wreck on a reef between Falmouth Habor and Martha's Vineyard. See my earlier comments about fog and tides. (Not so much fog but the tides are a huge issue. Diving there is a one tank proposition, jump in when the tide is about to turn and enjoy yourself but when your dive is over the tide is much too strong in the oposite dirrection to do another dive there.)
Give Mike a call and see what he has going on. I have no idea if he rents gear or what he charges if he does. My only connection to him is we dive with them a couple of times a year. I remember hearing Jerry (mate) saying they would run pretty much any day if there was enough people to make it worthwhile. I'm working evenings next week, if something interests you I might be talked into going so long as we're back at the dock early.
There is also another shop in Sandwich, AquaCenter, right by the power plant at th eastern end of the Cape Cod Canal and there used to be one in Buzzards Bay/Onset. I don't remember its name or know if it's still open but you can google it. Up in Plymouth (30 minutes north of Falmouth) is a great shop, Divers Market, and in Marshfield (20 minutes north of there) is North Atlantic Scuba. None of those shops have a boat, but Marshfield does have a nice shore dive called Brant Rock.
The best diving in this area is the other cape in Massachusetts, Cape Ann. It's a good 2 hour drive from Falmouth but there are a load of dive boats up there.
OK my fingers are getting tired.
Paulthenurse