Rich Coast Diving is on the main street in Playas del Coco and runs the two trips this coast is known for: the Catalinas, where manta and mobula rays pass between December and April, and the Bat Islands in Santa Rosa park, where bull sharks gather at the Big Scare.
The Bat Islands are an advanced dive — current, surge, an hour's boat ride each way — and they will ask about your logbook, which is a good sign rather than an annoyance.