We had lunch at Carmelita's. It is in the middle of several palapa restaurants. There were a couple of other very nice restaurants right next to Carmelita's. The food was very good and they had authentic Mexican coastal fare. The service was excellent and this restaurant was especially clean and well cared for.
The beach is almost too calm but you can swim there or take a banana boat ride. You can also rent ATVs and drive around on the sandy beach if that is your mood.
Most of all it as a VERY quiet place to escape the hubbub of Mazatlan, which is a very noisy busy shipping port more than it is a resort town.