I went to CR in July 1998 and it was wonderful! We stayed at El Ocotal on the Guanacaste coast. The diving was spectacular....best I've ever seen. In my opinion it beats the pants of Cayman Brac by far. We did Catalina and Bat Island too in addition to many other sites.
Yes, it's a 4 hour drive from San Jose to El Ocotal. It was worth it as we got to see the countryside and local color. We made several stops along the way and had a guide on our bus that explained everything too us....very enjoyable.
I would go again in a heartbeat....but there are other places on my list I haven't been yet....too soon to start repeating!!
The summer is the rainy season in CR and the best time to go. The countryside is more lush and the visibility is better. Keep in mind the visibility in CR (Atlantic side) is not crystal. The water is packed full of nutrients that feed the abundant life you'll find there. You have to give up excellent vis to get the incredible life. I wore a 3mm and was okay during the day dives, but was cold on the night dives. When I go again, I'll take a 5mm with me with a hooded vest just in case.
One afternoon we also did a canopy tour above the rainforest. IT was spectacular....a must do if you can fit it in. When we go back we plan to spend a bit more time there than a week and split up our time between diving and land based activities. My buddy and I are both kayakers and would like to do that in addition to the rainforest, volcano,...etc.