Jake11 and I agree on so many things and I can't comment on land activities in Costa Rica, but as for diving, I must ask jake11, "have you been diving out at the Atolls?".
I think the diving out at the Atolls in Belize is really really good and can be amazing (for the Caribbean )!! The local diving off of San Pedro is merely good (no not Cozumel good ) but it's not drift diving, at least not so's you'd notice. New divers will like it just fine especially with the large groupers, nurse sharks, moray eels, reef and bull shark sightings, occasional dolphins coming around to check out the divers, and the other day a manta ray! !