I dove in Costa Rica a few years back.  It was the most enjoyable
diving that I have done in years.  The visibility can be very low and
there is little to no coral but you do see Southern Stingrays, Spotted
Eagle Rays, a variety of eels, White-tip Reef Sharks, tons of Puffer
and Porcupine Fish, Octopus, turtles, Seahorses, lobsters, Nudibranchs
, etc.  I did a three-tank dive to Catalina Islands and a two tank dive
to bat island with rich coast divers
(http://www.richcoastdiving.com/).  On the Bat Island trip we saw parts
of bull sharks.  The visibility was low and the sharks were big so you
could not see all of them at one time.  On the Catalina Islands trip, I
saw schools of stingrays that went as far as I could see (visibility
was a little better  say 50-60 feet), spent over 12 minutes with a
giant manta ray, came within arm distance of three white-tip reef
sharks and saw lots of dolphins, and turtles on the way to the dive
sites.

The local dives were also fun.  On most dives I saw the list above but
on one dive we saw another school(?) of southern stingrays.

Diving was up in Guanacaste on the west coast. I stayed in Playa de
Coco at a very nice small place,  Pato Loco Inn.