MANTA Scuba Cabo
Manta has the highest reputation among dive operators in Cabo San Lucas for a reason - they're professional, safe, run very nice boats and offer a full-service PADI 5-star dive center.
I've dived with Manta twice in the last two years, and on both occasions the staff has been very fun and friendly. They also offer whale watching during the winter months (migrating humpbacks), plus dive trips to Gordo Banks and Cabo Pulmo. They will even take you snorkeling with whale sharks in La Paz during late summer.
Long story short - Manta is THE operator to dive with in Cabo!