I've never been diving in Cabo -just the other coast. Diving in Mexico seems to be feast or famine. the stuff I've done on the Mayan Riviera was pretty plain. The Cenote (inland fresh water sinkholes) were mind-blowing though. I wouldn't recommend them for a newbie though -you're diving in caves and need to have pretty good buoyancy control.
If you go to:
http://www.padi.com/scuba/locate-a-p...p/default.aspx
And enter cabo san lucas, you'll find a number of PADI Dive shops. PADI isn't the only certification body around but they are perhaps the largest and their standards are high and you will get good training. I'd suggest you google each of the shops listed and see what you can find in terms of a review.
A word of advice: You're running short on time but I'd STRONGLY recommend that you go to your local dive shop up here and take the theory part of the certification HERE. Why ? 1) It'll be far cheaper 2) It'll be in good english 3) It will leave you all your vacation time for diving a mojitos when you're in Cabo instead of sitting in a classroom.
This is called a "referral dive". So named because the PADI shop up here will do half your training (the theory) and then "refer" you to the shop down there for the open water dives you need to complete for certification.