Given the short timeframe of your trip - for 5 days I'd suggest Grand Cayman - 5+ hr. flight from STL thru Atlanta - or Curacao 7+ hrs. For 3 days you may want to consider the Florida Keys.
For Cayman you'll likely need/want to do some sort of refresher course the 1st day so a couple of options might be one of two AI resorts on GC - either Sunset House or Cobalt Coast. Both have easy entry shore dives off their property and also boat dives. And Stingray City is the "most fun you'll have diving in 15' of water" Sunset House is in south Georgetown so it's easy access to dining, shops etc. And there's several other easy access shore dives (Eden Rock, Devils Grotto, Foster's reef) within a few minutes drive.
Cobalt Coast has a long dock that takes you right out to a shore dive off their property. And Divetech is on-site, they do everything from beginner classes to tech/rebreather training. Turtle Reef nearby is my favorite shore dive - it's an easy entry off Sundivers ladder, short swim to a mini-wall just teeming with life that bottoms out at about 50' To the south there's often a huge school of Tarpon hovering in a cave(more of a depression) in the reef - moving slowly through them was surreal. Cobalt Coast is at the north end of Grand Cayman so has better access to diving on the North wall and is more all-inclusive. It's just a few minutes drive south to really good restaurants along Seven Mile Beach if you get tired of eating there.
Or stay at any # of nicer hotels along Seven Mile Beach and there are multiple dive ops that will pick you up and shuttle you to their boats for free. Many of them limit divers to 6-8 so will be able to spend more personal time with you. There's no shore diving along SMB - that's why it's such a nice beach. Everything you need to know can be found here:
http://www.caymanislands.ky/
The water clarity in Cayman can be unreal. Off the north wall at Big Tunnel I looked up at the boat from 90' and could clearly read the Scubapro "S" on the safety stop regulator hanging down next to the hang bar. Curacao isn't bad either.
Curacao has the same excellent diving as Bonaire but is more accessible. Three places to consider: Lions Dive, Habitat Curacao or Lodge Kura Hulanda. Lions Dive is just south of downtown Willemsted, in the Seaquarium resort area and has the main Ocean Encounters (largest diveop on Curacao) location about 50 yards away. Nice looking resort, beach bar etc. (never stayed there but we rode by it twice on a boat) and there's also a shore dive off their property. Ocean Encounters will require you to do a refresher course prior to diving with them most likely. Another nearby site, Tugboat is one of the best in that area. The boat is in 20' but the better part of that dive is the nearby pier and the wall around the point - just teeming with life.
Habitat Curacao is mid-island to the west. It's an AI resort with what I feel is one of the better dives just off their property. Simple entry off their dock and a short swim out to a reef that starts at 40' and goes beyond rec depths. We saw more eels there than anywhere else. They're currently involved in constructing a new diveshop and making some other changes so I'm not sure how much that will affect staying there in March.
Lodge Kura Hulanda is about as far west as you can go. It's a 45min. drive from the airport and is located in Westpunt - a sleepy little town. It's one of the Leading Small Hotels of the World, service is outstanding, suites are very upscale. It's a full AI due to location and the restaurant has amazing views since the resort is perched on cliffs above the ocean. Ocean Encounters West is the diveop there, good people, great facilities, showers, lockers etc. right at water level. A short walk down their divedock and you're on Playa Kalki/Alice in Wonderland - IMO - and many others - the best shore dive on Curacao. And arguably the best boat dive - Watamula is done twice a week from there.
Downside is the drive back to town for just about anything else not found there. There are 1/2 dozen or so local restaurants in Westpunt but not much else.
Nice thing about Curacao shore diving is that there's a lot of sites, most have an on-site dive operator for tank/weight rentals and almost all of the shore dives are simple entries either off dive docks or a sandy beach. Some of the western sites can be a little bit of a surface swim out to the drop but it's manageable. And you see a lot of stuff, more turtles than you can count, rays, seahorses, lots of tropical fish and amazing coral formations (Mushroom Forest) At one site, Varsenbaai, we dove an 80' deep pontoon wreck, found seahorses at 40' and swimming back in found turtles feeding in 8' just off the dive dock. The operator said they're always there. (3 years ago)
It's really underrated but I think it's easily one of the top 5 dive locations in the Caribbean. Curacao is a more populated island so there are casinos, nightlife and a famous shopping district in downtown Willemsted right on the water.
hth,
