I'm PADI certified and have gone diving in many parts of the world from Thailand to Brazil, but I'll admit I've never dived in CR. OTOH, I have looked into it and what I've read about it has not exactly lit my fires to try it there. The best place for diving in CR is definitely the Isla de Cocos way out in the Pacific, but that is not exactly a place you go for just a night or two. Usually its done as a multi-day package aboard a dive boat as it takes time to get out there and there are no other facilities once you're there. If you like reef diving and don't want to commit to an expensive and extensive multi-day dive trip then the southern caribbean coast will probably be your best bet for diving in CR. The NW Guanacaste area, particularly the Papagayo Bay area around Playas Hermosa, Cocos and Ocotal is also a major dive center for CR. You won't find any real reefs there, rather mostly cliff dives with lots of pelagics (whales, sharks, rays etc.), but you'll find more places offering scuba lessons, equipment rentals, etc. One great link that Greenfangs left out is to my more extensive post on CR diving
http://www.costaricaticas.com/phpBB2/viewtopic.php?p=34847&highlight=diving#34847
BTW, if you're going to go diving in CR then in general late April may be the best time to do it. Poor visibility is one of the most frequently reported problems with diving in CR. And one of the prime causes of that is silt and muck carrying runoff from the mountains during the rainy season. With late April being at the very tail end of the dry season that factor should be at its most minimal. OTOH, I'm not so sure when would be the best time in terms of the wildlife (whales, turtles and such).
One last comment, if I were going on a scuba vacation and especially if I were going to learn how to dive, then I'd probably go to the Bay Islands in northern Honduras or maybe one of the Cayes in Belize rather than CR. The dive conditions and the quality of the reefs are much better there than in CR and the cost of instruction, boat fees and equipment will be much less expensive (especially in Honduras) yet still very safe and professional. Unfortunately, the P4P there won't be nearly as good as what you will find in SJ or Jaco, but then you can't always have everything.
Another possibility with good mongering opportunities right at the coast but with probably much better conditions than around Jaco might be the northern coast of the Dominican Republic (Sosua). It really all comes down to your priorities and preferences.