A good shallow shore dive is at Chakanaub. They also have wimming with Dolphins, for your girlfriend. It’s a $10 taxi ride, plus there is the park admission and equipment/tank rentals. Beautiful dive down to about 45 feet max.
I have also used Pepe’s Scuba at the Coral Princess hotel. You still have to take a Taxi to the hotel, but they will drop you off at a pier by the cruise ship terminal. Fast, small boats. Perssonalized service. I would definately recommend. They always responded to e-mails within 24 hours while working out the details.