I've had the privilege to have fished in many really cool locations in my years. However, the best fishing by far I've ever done was central Pacific coast of Costa Rica, from mid-October through early March. Sailfish so big they're difficult to get onto the transom for a photo. Marlin that...
Also, remember that the average size of the sailfish in CR is about twice what you'd see in Florida gulf waters, or off the Yucatan. They're definitely a thrill to catch, you'll very likely catch a lot of them too.
Just do your research. Boats and captains are migratory, many of them work up and down the coast depending on what's running and time of year. Check in with Los Suenos marina ( www.lossuenos.com/deepsea-fishing/ ), they have an extensive fleet and because they're so high profile (tournaments...
Then September may be a bit early, the closer you get to October-March the better chances of hooking into a marlin. Also, if you're wanting this, then be sure to go for the bigger boat, and check out the crew.
Also, what kind of fishing are you wanting to do? If you want the excitement of seeing sailfish jump, you can catch LOTS of them (I caught over 20 in one trip) and they like to put on a show for you. But after a while they get boring. Down there they call them "Sea turds" because they're not...
I've been to CR 100+ times, even married a tica. ;) I've been big-game fishing all over this orb and found the Pacific coast of CR during the September - March months to be the best fishing I've ever done, by far. A couple of things to look out for, though: Sails run strong at that time, but...
Caught this little guy in blue water off central Pacific (out of Los Suenos) Costa Rica, March. I go to CR almost every month (have fiance and place there) and although I don't fish as much as I used to, it is the best fishing I've ever done anywhere on the globe. Hands down.