There is some limited scuba diving on the Caribbean coast, but very little compared to the Pacific side, which is a popular dive destination. There are many dive operations all along the Pacific coast. The Caribbean side tends to be better suited for surfing, with sloping beaches. Inshore waters are usually murky, from wave action and muddy river run off. I'm told there are nice reefs well offshore, but when I was there 15 years ago all the diving was on the west coast, which has some spectacular sites, lots of fish, clear water, and many reefs. I enjoyed the Caribbean side's jungles and wildlife, and it was much less touristy when I was there. I didn't see much in the way of blue Caribbean water. Divers seem to strongly favor the Pacific side. If I were going I'd check a few travel guides and scuba vacation operations. Costa Rica is a very popular place, and there must be tons of information available.