My dad and I (60s and 20s) get to go diving about once a year or so, and we've been to a number of dive shops in different countries, (we've gone to Costa Rica twice before, but it was about 10 years ago, way before either of us were even certified) and Rocket Frog Divers has the best staff and service I have experienced as a tourist.
The dive shop is close by to Father Rooster, and is one of the nicest I've seen. It's not huge, but well-organized and an open, friendly environment. Their equipment is very organized, and their boats are up to date and allow for both smaller and larger groups. (Our group was only about 3 people).
Rocket Frog offers a wide variety of dive spots, both local and to the Catalinas/Bats, depending on weather, experience, and just where you would like to go. While I could go on and on about the actual diving, (which was great!) what really makes this a must is their staff and their dedication to great service. Everyone was very friendly, personable, knowledgable, and really went out of their way to make our trip so memorable. We felt like we really got to know the staff well, even as tourists just there for a few days.
Rocket Frog is only about 2 years old, but I would not be surprised if a few years down the road, word-of-mouth spreads, and they become even more prominent in Ocotal. They really deserve it, and I hope I can come visit again!