I'm sure the Westin Regina runs a shuttle bus, but if you can rent a car, the freedom of having the car is great! If you want to slum it, you can take the local bus (which looks pretty modern). I'm told bus fare runs around 10-20 pesos.
The Costco is a major culture shock
Best places to shop (unless you want the quantity in Costco) are actually CCC & Soriana. My wife prefers Soriana. Think Super-Walmart, but in Mexico. They're in Cabo itself (the Costco is on the Hwy about km 4 or 5).
I got certified & did a dive with Baja Dive (in Plaza Las Glorias I-4, near Solomon's Landing & Captain Tony's). I had a great time there; it's not as fancy as Cabo Acuadeportes (at Chileno Beach & Hacienda); but Paco (dive instructor) & Victor (dive guide) are great guys; very nice, easygoing. Their dive boat is not the biggest, but you won't be diving in a big crowd.
You should try to do a guided trip to Cabo Pulmo. We did a snorkel trip on our own to Cabo Pulmo, and were somewhat disappointed. Paco & Victor told us that the best diving & snorkeling in Cabo Pulmo are from boats... next time.
If you want restaurant recommendations (or where to park for free), send me a PM. We were just there end-of-May. August will be hotter, but water should be warmer & visibility better.