Hi,
Even though Puerto Vallarta in not known for its shore diving, there are a couple of alternatives that I can suggest:
One is Mismaloya, as mentioned in the previous postings.
The other is right accross from Los Arcos, just before you get to Mismaloya. This is a more adventursome option, as it requires climbing down the mountain some 100 feet after parking next to the highway. This is only recommended during the summer and fall months when the ocean is calm, as you can get some pretty strong waves breaking at the beach during other times. I'll recommend to do this dive conservateivly, since if you need assistance you'll have to rely on the good-will of one of the many boats traveling in the area, as there's usually nobode else at the rocky beach where you'll be diving from.
By the way, our dive shop is located in the Marina, in case you need any assistance with your boat dives.
Cheers!