My wife and I have done a fair amount of diving in Costa Rica. There is no time when you can count on diving at all on the Caribbean coast, it rains a lot there and there really is no dry season. When it rains a significant amount there is no visibility at all. If there is a week of good weather the diving is really nice, but as often as not we have not even been able to dive there.
Guanacaste is where most of the diving is. It can be fantastic, but more than 30-40 feet of visibility is very rare. There is a lot to see and you will enjoy the diving.
My favorite diving there is Isla Del Cano, on the southern part of the Pacific coast, also my favorite part of the country. The vis is better there than anywhere else we have been in CR. There is a lot to see as in Guanacaste, but it is much less developed in the south and it is green and lush unlike arid Guanacaste. There are no big resorts down south, mostly small bed and breakfasts, lots of houses to rent, some of them not too badly priced. The closest point to Isla Del Cano is Drake Bay from which it is only about ten miles. We have stayed at Corcovado Adventures Tent Camp and really like it. It sounds pretty expensive, but includes three meals a day because there is nowhere else to eat. We really like the area around Ojochal and Uvita a little farther north. The only problem with that is it is a thirty or forty mile boat trip each way. In Uvita Mad About Diving is a good outfit. We have used Mystic Dive Center in Ojochal as well and were much happier with Mad About Diving.
We have heard that the diving in the Bat Islands is great but we cannot get there when they are doing that trip.
I would recommend January or February for any of the Pacific sites except the Bat Islands which they only dive during the rainy season when the seas are calmer.
If you are interested, I am hoping to take a group next January for a trip focused on diving, but with a bunch of other fun things as well. I have seven people with strong interest now and I am hoping for twelve. If you have any interest, send me a private message.
Earl