I've traveled to Costa Rica many times in the last 7 years and as others have said, the diving on the Caribbean side is not very great. Lots of rivers dumping sediment and it rains more nights than most. As far as dive ops go Reef Runners seems to be the main choice. There are 2 other ops in town but one never seems to be open and the other is so busy doing other tour operator activities I'm not sure how professional the dive op is.
If the Caribebean is the side you'd rather be on I would suggest going down to Bocas del Toro in Panama and diving there. You'll have a lot more to choose from as far as types of diving and locales. It's about a 2-3 hour journey and there are multiple tour operators that will take you over from Puerto Viejo and help somewhat with the border crossing. Then come back to the Puerto Viejo/Manzanillo area (If you have a vehicle try somewhere near Punta Uva) for a little R&R.
Oh, and the part about, "Panama is less crowded, has less crime, more friendly and less expensive than Costa Rica." not true. Maybe 10 years ago but the 2 are so close together that the biggest thing you'll notice is being able to use dollars in Panama. Every thing else, especially in Bocas is going to be right on par with Costa Rica.