Where are you going to be at?
Are you scuba cerified? If not and you want to be you can get it in 2 weeks there. Try Scuba Panama in Panama City. Great little shop (at least it was in 90) and they had a great place to dive on the Atlantic side of the country. I found that was the best diving conditions I have every been in. Great water and critter populations.
The dive site was in walking distance to an old fort and you can walk the fortifications and see some of the old cannon still on the wall.
Frankly I found it to be more comfortable to be in Panama than in Mexico, and that was right after we had invaded it. Great people, inexpensive food and fantastic diving.