Do not miss the Bat Islands - best diving in the area IMO. Sharks, rays and bigger schools of fish - exactly what you asked about. You will be there 'in Bats season' so you shouldn't have trouble booking trips with any op. You may even have the chance to do an overnight camping trip to Bats (rare, but offered in the summer) to get some more dives in and explore some lesser dived sites.
I wasn't blown away with the Cat Islands (all depends what one is comparing them to) but you should dive them too to have an opinion. YMMV.
There are some nice dive sites right there.
Divescover - Playa del Coco has this to say ...
This area is great for diving with the big stuff such as reef and bull sharks, manta and eagle rays, huge schools of fish and large pelagic fish to the small macro critters such as nudibranchs. The striking underwater landscape is made up mostly of volcanic pinnacles with great visibility of up to 25 metres and average water temperatures are a mild 26 C-30 C; 20 C if you dip down into the thermocline which is a mix of cold and warm water.
Don't know about reef sharks, but there are bulls and white tips. Bulls only at Bat Isl. Possibility of a black tip shark if you are lucky. All sorts of pelagic rays, including manta, spotted eagle, mobula, mustard and bat rays, plus the bottom dwelling rays. All the local sites (non Cats, non Bats) - which is the majority of the diving done - I would describe as mostly boulders on sloping bottoms, not "pinnacles".
That "up to" is very important. If you have done some research you will realize that viz is typically much less than 25m. You might want to search here under Ocotal as well as Playa del Coco for recent diver reports. Temperatures above the thermocline can be much lower than 26 C, though the summer months are supposed to offer the warmest water generally. We had 3-5 degree F drops between dives and between consecutive dive days in Dec/Jan and were told this was common year round, as well as significant vis changes in sites that were very close to each other. Conditions can change rapidly and unexpectedly.
After the rain, the viz is worse due to particulate runoff.
I was actually told by both of the most senior dive ops that unlike in most parts of the world vis in the rainy season is
better in Ocotal/Playa del Coco than in the dry season. They will give you a detailed explanation of why when you get there.
Hope you get some great diving in - let us know how it went.
