We dove and stayed in La Paz Sep-07. It was great, with the sea lions being the highlight. We dove with Club Cantamar and stayed 15 minutes away at El Moro. There is plenty of info on Club Cantamar on this board. El Moro was very nice with a great staff.
The town is not too touristy as you say. There are plenty of good restaurants, with great seafood since it's a fishing town. We walked around it at night and didn't have any problem. I have taught myself a fair amount of Spanish but I think you'd be okay without it. At a couple restaurants our waiters only spoke Spanish but food is pretty easy I think. Restaurants are not particularly cheap, maybe a little lower than Detroit or Philly.
The water can be a little cold sometimes in 3mm suits but that's all we have besides our 7mms that we use at home. Not really a problem but in July it will be a little cooler than in September. Air temp in Sept is around 100F, with July almost as high.
We would like to go back. The diving is really good and was our first time in Pacific waters so different animals than the Caribbean. We also snorkeled with a whale shark which was great. If you have a camera, at least take it on the sea lion dives. A young sea lion literally draped himself over my head and back, his chin resting on the top of my head, and body on my tank, parallel to me. Unfortunately I am the photographer, and was not able to transfer the camera to my wife/buddy in time for a good shot. So we missed 'brochure material'. One thing to check on is if the sea lions are old enough yet to hang out with you by July. I was thinking along the lines of August/September since they are born mostly in April.
Either way, I think you will like it.
