I went there about 2 years ago and had a great time. One of the best bargains ever! There weren't many Americans, mostly Europeans, but Punta Cana has been trying to market more towards Americans.
The beaches were great and the hotel staff was great as well. We did not get to go out of the hotel complex much so I didn't have an opportunity to mingle with the locals. It wasn't recommended outside the complex at night because of safety concerns
I went there before I was certified and didn't see too much scuba activity, but I think things have changed over the last two years. I did go snorkeling off of jetski tour of the reefs and they were fairly healthy. This was the only time I left the hotel complex.
Some people I know recently went to Punta Cana and didnt like it so much because of where they were staying. I stayed at the Rio Palace and it was great. A top-notch hotel/resort and a steal.
I would love to go to Punta Cana again someday and I would defiantly take my scuba gear with me when I do. Hopefully it will remain a little "quiet" for the obnoxious Americans and remain unspoiled.
I highly recommend it, if you stay at the right hotels. This was two years ago though. I think a lot has changed.