I'd look at it a different way, with only a few days you don't want to spend them traveling. Little Cayman will require at least a day each way since you fly into Grand Cayman first so allowing for a no-fly day, you could be limited to 2-3 dive days on a 5 day trip. Some better options might be:
Boston to Grand Cayman - there's a USAirways flight that arrives there at 11:45AM - leaves at 5AM. The flight back is about the same.
Sunset House has single rates and both a 3 and 5 day pkg. so maybe they offer 4 days also. Or there's Comfort Suites or Sunshine Suites on Seven Mile Beach - most dive operators pick up at either. Many operators do 2AM + 2PM dives if there's demand, some do Stingray City instead in the afternoon. We did two boat dives every morning - back well before noon - then shore dove in the afternoons. Shore dives are at Turtle Reef, Lighthouse Pt. Cobalt Coast, Eden Rock/Devils Grotto downtown or Sunset House. Not the cheapest place to go though - everything is expensive - boat dives typically run 2/$100+ I have a sense that Lighthouse Pt. might be a good macro dive. Many operators will go with 4 divers - most less than 8. So it doesn't take much to fill a trip.
BOS to St. Thomas - There's a direct Jet Blue flight - 9AM arrives at 1PM but you have to fly American on the return - takes longer. There's also a couple of dive operators downtown - Admiralty or Blue Island - I think both pick you up. One is near a hotel also. As well as there's a Best Western waterfront, I think a Holiday Inn also. The Marriott is pretty nice also but larger just outside of town. Or Bolongo Bay, they have St. Thomas Diving Club onsite. From the Marriott STDC is probably the closest dive operator also. With a car you can also access shore diving at St. John via car ferry daily. It's even possible to go over the the BVI's - a couple of St. John ops do it during the week. The ferries run early enough so you can get there and back to meet their boat.
BOS to Providenciales, (Provo) Turks/Caicos is also one stop - same 5AM departure, arrives around 12:30. Provo has great macro diving, a friend shot a bunch of it there. All sorts of tiny fish, nudis, small crabs etc. There's a Comfort Suites in Grace Bay also and dive operators pick up for boat dives. Most of that area is walkable at night, shops, restaurants etc. There's a couple malls in Grace Bay and the casino will pick you up free. Dive ops either dive off Grace Bay/NW Point locally or go to the better diving about an hour south at French Cay/West Caicos. They pick up at most of the hotels, shuttle you by van to the south side marina, it's a 45min. boat ride, 2 dives + lunch. You'll be back about 2:30. Add a 3rd dive and be back at 4:30. So it's not quite 4 dives per day.
I personally think Provo diving is slightly better than Grand Cayman. No shorediving though.
Those would be my choices, I dove off St. Thomas and the BVI's the week of April 20th in a t-shirt and board shorts. Cayman and Provo will be similar. Maybe a little cooler at depth still since the wall off Provo is 6000' deep. I know you said macro is preferred but there's a lot of sharks, turtles, rays, cuda, there also.
Might also check Nassau also, all boat diving. Stuart Coves picks up daily and does 4 dives/day. And shark feeds.
The other option is probably West Palm Beach or Ft. Lauderdale. For Macro in that area Blue Heron Bridge seems to have all sorts of interesting things - in really shallow water. Lots of boat options also. IDK how warm it is yet being the Atlantic.
