In Costa Rica you actually have 3 dive sites that can compete to be the best. (Excluding Isla del Coco as it is a different cost and organization).
1. Isla del Caño
2. Islas Santa Catalina
3. Islas Murciélagos - Bat Islands
Starting from behind - The Bat Islands and Catalina are very interesting locations where, as previous commenters have mentioned, you can meet bull sharks, which is a cool attraction in itself. Unfortunately, conditions can be problematic, which are usually considered quite challenging - not the best visibility and a moderate to strong current. Especially during the rainy season.
Isla del Cano, on the other hand, is considered Isla del Coco's smaller sister. Conditions, of course, are also variable, but due to its remoteness from the mainland, rainfall and numerous rivers on the mainland do not significantly affect conditions around the island. However, what you need to know, Isla del Cano is a biological reserve where the daily number of tourists (including divers) is strictly limited. Diving is allowed only when accompanied by a licensed guide, and the number of dive points is limited due to island conservation.
Dive training location
As for your training, Isla del Cano would probably be the best choice for several reasons. First, conditions are usually favorable, even during the rainy season. Various dive sites with different characteristics are offered at the island, which will be helpful for your dives on the advanced course. Navigation, deep, search and recovery, etc. - there are cool dive sites for all this.
You can get to the island actually from three places - Uvita, Sierpe, Drake. And here Uvita has the best location and is well connected to the capital or other parts of the country. Getting to Sierpe requires passing through Uvita and an additional hour of travel. Drake is even more remote and requires getting to Sierpe and then by boat to Drake Bay. Alternatively by land if you have a 4x4 vehicle.
A larger study of diving in Costa Rica can be found here:
Scuba Diving in Costa Rica | The Best Places for Divers
Dive schools
As for dive schools, there are more than a dozen in the area, but most focus on serving certified divers, and if I were looking for training I would choose those with 5-star status, because that means they have experience in training. It's also always a good idea to ask at the dive center who your instructor will be and take a look at their training experience.
---
I should also add that I may be considered biased because I personally work not only in the diving industry but also in this very area, but let me add here some photos from our Instagram (
@costaricadivers) and from Isla del Cano:
![sh1.jpg sh1.jpg](https://scubaboard.com/community/data/attachments/790/790667-da2a23412361a7f700461f4a48e1fefd.jpg)
![w2.jpg w2.jpg](https://scubaboard.com/community/data/attachments/790/790668-d5ee231ffb2f3436d2a2690628ef8c7d.jpg)
![9.jpg 9.jpg](https://scubaboard.com/community/data/attachments/790/790669-7ed4e5b48f566f10cb047e474c11da55.jpg)
If you have any questions I will try to help. Pura Vida