I'll answer generically, hopefully others will also chime in.
Where in "Cabo" will you be staying? The region described as Cabo can be construed as anywhere from Cabo San Lucas to San Jose del Cabo, and accessibility to certain features will differ.
We've been twice, both times staying close to the marina (Cabo San Lucas proper). Unless you plan to travel widely throughout the region, a rental car is not necessary. We get a van from the airport to the hotel, and then the general marina are is walkable.
If you stay up along the Corridor, you may or may not need a car (not that familiar with cabs in this region, although they are very visible on the streets).
In terms of drinking and dining options, if you are close to the marina, you'll find just about everything you need.
In terms of diving, we've used Manta Scuba both trips, and each time have done a trip to Land's End and another up the Corridor; and then one specialty trip - first year to La Paz, second year to Cabo Pulmo. The specialty trips were also through Manta. Manta is at the marina, however there are other dive shops there, as well as on up the Corridor.
