PADI certification on Utila Bay Islands Honduras, is $300 including dorm lodging. Getting there is not easy, most include a visit to Copan Ruins in Honduras, via Guatemala City. Allow 1 week for visiting the Bay islands/Diving when all said and done.
Take your PADI course online prior, then classroom time on vacation reduced and easier.
You might want read a recent trip report on San Blas Islands, its not cheap and not very easy to reach. I have not been though. Read the thread with a recent visitor reporting thier trip.
https://www.lonelyplanet.com/thorntree/forums/americas-central-america/panama/san-blas-islands-154bda7b-096f-4829-811c-2224997f241f
I would skip Panama, and take in the nature/wildlife & National Parks of Costa Rica, as that is what it has to offer that is best, besides great surfing and beaches on two coast.
Yes get a guidebook...
Get some Spanish immersion/basics under your belt with two months, and a lot of transport/transit, will make life easier and trip more fun...
Adventure Travel to Colombia, Costa Rica, Nicaragua, Honduras, Guatemala, Mexico, Thailand, Burma, Laos, Cambodia, Vietnam, China, South Africa, Morocco, Turkey, EU, USA National Parks, enjoying culture, cuisine, motorcycling, scuba diving, surfing, sailing, rafting, hiking, fishing, camping, nature, wildlife. Get a Guidebook, and get lost!