The logistics of getting to the BVI's make that a little rough since you'd have to fly into St. Thomas from the DC area plus ferry over. The fast ferries run hourly so it's not as bad as it sounds. May not be real warm there yet though next month - the east end of the BVI's is technically in the Atlantic. One other gotcha about the previous suggestion is that DiveBVI is an SSI shop and I don't think PADI will accept a reverse crossover. Otherwise they're a great outfit. But they're on Virgin Gorda which has less ferry service - since you're fighting the clock. If you did want to do the BVI's, I'd go for a PADI shop on Tortola - they're all listed here: bviscuba.org. I suppose St Thomas itself is an option also. It's a little colder there though - currently 79
o. Bolongo Bay/St. Thomas Diving Club is known to work with beginners. And they have a beach there for classes.
I'd look warmer. It may not sound like much but a few degrees might matter if you can't find anything but rental shorties - which is common. Aruba is a 7hr. flight each way from BWI. Water temp is 80
o right now. I used 4/4-4/8 as dates. Nice thing about the return flight is it's at 4:50PM so you can dive all day Mon/Tues.
Grand Cayman looks like about 5.5 hrs. out/ 7+ back total with a stop in MIA - from DCA. for $495. Water temp is 81
o currently. It's pricey there but one advantage is there must be almost 2 dozen PADI shops on the island for cert dives. Save a little money and stay off Seven Mile Beach - Comfort Suites or Sunshine Sutes are options. Ambassador Divers is also at Sunshine Suites - I recommend them regularly.
Also there's several easy entry, excellent shallow shore dives either on the NW side if you have time. If the kids are old enough to leave alone, you could wall dive also. All the shops:
Cayman Islands Tourism Association - Things To Do In Cayman, Cayman Culture,Cayman Islands Watersports After the kids are certified they can dive Stingray City (15' deep) or the Kittiwake wreck also. It's a big boat, shallow - 60' tall sitting on a 70' bottom and escorted. Stingray City trips are generally in the afternoon so that may work out also.
I used April 4-8th for this - figuring you may not find much open on Easter so two days for diving. To look at other destinations, use this surfing website for water temperatures.
Surf Forecast and Surf Reports Worldwide | Surf-forecast.com
I use
Matrix - ITA Software by Google for flights - much of the same data is available on Google Flights - just not as much tweakable detail.
I don't think you're really going to find anything warm in the Keys/SE Florida yet.
---------- Post added March 14th, 2015 at 08:06 AM ----------
Factor in that the better diving is not in San Juan but SE in Fajardo or south in La Parguera(sp?) San Juan is pretty murky at times.
One of the shops does teach at a shallow beach site a little to the east of the cruise port in San Juan. I think there's "Dogs" in the shop name.