unless there's a hurricane, I'd steer clear of the Caribbean in August. And as this is your first surf trip, I'd definitely recommend Costa Rica, which is a lot easier to navigate than Nica or other countries down there. Easier to get around, more infrastructure, generally a bit safer, etc.
the winter months are when you want to go to Puerto Rico. stay on the west coast, lots of night life and cool things to do. I went with the wife and she wanted to stay in San Juan so I had to drive a little ways for good surf. how ever there is a awesome little break called aviones like 15 min from SanJuan right behind the airport. people are great foods great surfers are cool. just remember your the visitor and give respect. the boogie boarders can get a little rough, they will sit on the inside of you and try to take every wave.but I highly recommend Puerto rico I went in March and I plan on making it a yearly trip.