Lead, it really depends on what you are looking for. Big dive resort? Small personalised dive shop? Are you looking to take courses? Recreational diving and/or technical diving? Are you going to be renting gear? There really are lots of options and many of the folk here will give you good recommendations!
For my part, I can only recommend the dive shop I did my DM internship with, the Divedojo. It's small and personalised (they can acomodate around 10 divers I believe).They can teach anything from Bubblemaker to semi-closed rebreather. They'll take you to do the type of diving you want to do (you just need to tell them what you want), as long as it is within your abilities and the conditions permit (the currents at some sites can be wicked on an ebbing tide!). For an example, I did several night dives on the Alma Jane wreck while I was there. It's a great wreck dive during the day, but at night, it is fabulous. And they are one of the only (if not THE only) shop that does that dive at night.
They aren't the cheapest in Sabang, but they are a relatively new shop (1.5 years) that was built with "only the best" in mind. All their gear, filling station and setup is top of the line. The rentals are brand newand high end. When you rent, they throw in a dive computer. The schoolroom has computers and a giant screen with DVD. They have an Italian Esspresso machine for free coffee (the best on the beach!
), they have free wireless internet, and free drinks (Bottled water, tea, coffee and soda during the day, and after the last dive, beer and other good stuff). Plus, they are a great crazy fun bunch of divers that love what they do. When you factor all that in, they aren't that expensive anymore.
This said, they aren't the only dive operation in Sabang and depending on what you are looking for, they, or another operation will fit your needs! let us know what type of operation (or experience) you are looking for, and people here will gladly try to help steer you to a good place for you!
Oh yeah, about ATM machines... there aren't any (as Chip said). But there is a Tourist change center which will let you take out Pesos on your credit cards. That's the way I did it while there...
Hope this helps a bit!