The diving in Costa Rica definately depends on what time of the year you plan to dive.
We love diving all year round here, on the North-Western Pacific Coast (area of Playas del Coco) with easy access to the Catalina Islands, the Bat Islands and numerous local dive sites giving you the widest choice of dive sites in Costa Rica with fair conditions the whole year round.
If your main criteria is warm, clear and calm waters, I think this would be your best choice for Costa Rica:
March through September: Gulf of Papagayo (Playas del Coco)
September through October: South Carribean (Puerto Viejo), although it seems that for Caribbean diving it's much better from Mexico down to Honduras and in the Caribbean Islands.
November though February: South Pacific (Bahia Drake, even if it's kind of hard to get there by road and then by boat, this is the closest location to Cano Island and it's a fantastic destination close to the Corcovado National Park that you can visit by boat on a day trip (make sure to visit the Sirena Ranger Station with less people and therefore better chances to see wild life and take a naturalist guide with you, they will see a sloth where you do not even think of an animal...). Anyway, Cano Island can be dove from anywhere starting Jaco to the south.
Well, as conditions can change daily, don't take this information for granted, it's just an estimation based on mine and other traveller's experiences.
As for me, I have never dove a better dive site in Costa Rica than Black Rock at the Bats, which often has more fish than water...;-D and it's rare to find other spots around the world to dive with Bull Sharks! The best season for them is just about to start...
Pura Vida!