From the messages I've seen here, my experience and the experience of a couple friends the diving on the pacific coast of CR varies with the seasons (if not weeks). I was there in November last year and absolutely loved the diving. Friends of my were there in August or Sept and the viz was poor so they didn't care for it. (they didn't do the Catalina or Bat islands, which I did and that could make a big difference.)
The previous owner of Rich Coast diving indicated she felt Oct, Nov were the best times to dive there, the water was warm and the viz was usually pretty good.
On my trip the viz changed from site to site and day to day. 20' on the low end, 80' on the good ones. I'd say the average was closer to 60'. As someone else mentioned, you won't find much if any coral there. On the local dives we saw, reef sharks, eagle rays (at least 2 on each dive), many free swimming eels, seahorses, clown shrimp. On the trips to the bats and catalina we saw bull sharks, HUGE mantas, schools of devil rays, balls of bait fish larger than a school bus.
You can see some of my pics here if you are interested.
http://www.toptown.com/divephotos/costarica/
The water is definitely colder than the caribbean, with 5-10 degree thermoclines on most dives. I wore a 3mm and got cold on a few dives (i was in utila last week and didn't wear a wetsuit for comparison).
We didn't stay at a "dive resort", we did get a package deal from Rich Coast and stayed at La Puerta Del Sol. The hotel is a very short walk from the dive shop, town and the beach, and they handled the gear each day so all we had to do is walk to the beach. There are a ton of great places to eat in town (and most are reasonably priced), the people are more friendly than many of the caribbean islands I've been too.
Overall, I loved it, and even though i usually prefer to always go somewhere new, we have already decided to go back again this year. Hope some of this was helpful