It is neither necessary nor desireable to pre arrange daily activities (like diving for example) in advance of your trip. This is especially so in the slow season, when bad weather may influence what you decide to do.
My advice is just chill til you get to AC, then go to a few of the dive operators you have seen recommended on the board that are near your hotel, condo or whatever, and see if their trips interest you.
Many of them will give you a reduced rate for multiple trips and a discount because you are booking direct and they are not paying an agent's commission, especially if you are paying CASH.
We usually go with our friend Gerry Ack, at Sol Caribe (next to the Paradise Hotel pier) for day trips. If you want an overnight trip to the outer atolls, there are several to choose from, but you need to be there to check out the schedule and the weather.
When you pre book activities with travel agents you risk being stuck when the weather is bad. That happen to us the very first time we went to AC and we never did that again.
There are many reliable operators competing for your business and you need not worry about needing to book in advance.
Susan Guberman-Garcia, Attorney at Law. Phone: 510-792-2639
Fax/Voicemail:: 510-405-2016 Email: firstname.lastname@example.org