I was in Playa del Coco in August of 2004 and did 6 dives. Visibility was pretty good and diving was pretty good too. Losts of variety of marine life - rays, eels, sharks, scorpion fish, octopus and the regular reef creatures as well as bigger fish. I dove 4 times with Dive Ocotal (I think that's the name, it's right at the Ocotal Resort) and twice with Deep Blue Adventure Diving from Coco. Both shops were very good. I did the Bat Islands - unfortunately saw only one bull shark for several seconds. The currents at Bat island are strong and it is pretty far out (1.15 minutes boat ride), but it was a lot of fun - we saw a lot of eagle rays on our second dive. I also did the Catalina Island, but did not see any mantas. Local dives are much closer and pretty good.
They do call it winter in August and it was puzzling to me to since it's above equator, but that is their rainy season and The schools have of in December and January just like we do during summer. I guess that's why they call December and January (dry season) summer. Don't worry, even during wet season it does not rain a lot in Guanacaste. hope this helps, If you want more info let me know.