You can't go wrong scuba diving in the Caribbean, the real question should be where shouldn't I scuba dive, that one would be harder to answer anyway. Costa Rica is not the best place to go scuba diving but like I said you can't really go wrong scuba diving anywhere in the Caribbean.
There are many places in Costa Rica that you can go to on an 'Adventure Tour', but you can also just go down and scuba dive anywhere and enjoy yourself. The difference will be that the tours will bring you to the best places, while you'll have to be content diving wherever you happen to be, good place or not. As long as you're near a reef you'll see some beautiful things under the water. Professionals may help avoid shark infested waters too, but if you ask around I'm sure you'll find out if it's safe at any given spot.
If you go to an all inclusive resort they will provide a short course in scuba diving should you need it, as well as the gear. They'll also probably find a place that's not infested with dangerous sea creatures as well.
Here's a link to an expensive adventure tour that provides scuba diving, white water rafting, rainforest tours and a few other things.
http://travel.iexplore.com/ksmap/Costa+Rica+Scuba+Diving
Check out these sites to find cheep vacations to Costa Rica.
www.selloffvacations.com
www.lastminuteclub.com
