Puerto Marques is a sheltered bay south of Acapulco's main curve, long used for swimming, seafood lunches, and boat outings. The water is usually calmer than the open Pacific beaches, and the setting has a more enclosed, local feel.
It is a useful change of pace from the hotel zone and cliff-diving circuit. Go during the day, choose busy beach areas, and treat it as a relaxed bay stop rather than a polished resort enclave.