I've fished out of Guanamar Resort at Carillo just around the point from Samara. There are lots of panga commercial fishermen on the beaches so if you know some Spanish and are an experienced angler with your own gear you can save some money with the locals. Howver, I usually advise newcomers to play it safe and use an established guide service as Kyle noted above. Some of the beaches (e.g. Carillo) have been designated by Costa Rica for indigenous people only so there are few hotels near the water (just the fishing camps). The beaches are not pristine white sand either. By the way, when I was there we had to run all the way to Punta Blanca to find any billfish.
I fish out of Carillo every year - this will be my 6th year running. I fish with Rick Ruhlow who runs the Kingfisher. He runs a great operation and I highly recommend him (will be on his boat on Feb 28th and March 2nd - can't wait). Also agree with the post above - Richard on the Gamefisher has a great reputation. A couple of other boats to try are Flamingo II with Capt Gene Watson (fished him in 2008 and had a great day), and Wet Ass with Sonny Kocsis (Sonny was just featured in Saltwater Sportsman magazine for a story about fishing out of Carillo). Samara/Carillo has become our go to family vacation spot. Its tough to beat the fishing/beach combo it offers. People are amazingly friendly.