If you say "diving" and "Thailand" Koh Tao is one of the places in Thailand to visit. Koh Tao is a small island in the gulf of Thailand and famous for its dive spots. Most of these spots are close to the island and in shallow waters so it's the perfect place to get you PADI and explore the ocean.
The main beach of Koh Tao is Sairee beach. The place is packed with hostels and guesthouses and lots of places to eat and drink. Every accommodation can hook you up with a dive school so you can get an introduction to diving or enlist in a 4 day course to get your PADI.
Getting your PADI is cheap in Koh Tao. For around 8k to 10k bht you can join a 4 day course which teaches you the basics in diving. And you will explore the underwater world directly.
Boats will bring you to diving spots around the island. Lots of coral and fish can be spotted and the water is clear so visibility is great. The below video shows white rock.From the surface you can see the bottom of the ocean which is 10 meters below. Slowly swimming to white rock of which the top is just below the surface. Lots of divers at the bottom practicing for their PADI and having fun.
One of the places providing quality PADI courses in different languages is coral grand. It's a nice resort at the beach but also has a large diving school with good facilities. One of their dive masters is a Dutchmen called Perry, he is the nicest person ever. Just check out their website for more information.