If you are looking for resort-type entertainment, such as live music, animation 'teams' and beach activities, the El Cid Marina is not the best choice - any of the other El Cid locations in Mazatlan or other larger resorts in the Golden Zone or the Riu would be better. The El Cid Marina is gorgeous, but more sedate, with minimal planned activities - depending on how busy it is. When we stayed there last March it was relatively quiet (just what we wanted) with infrequent pool volleyball in the shallow heated pool, and some live music (maybe a guy on a keyboard or a Mexican quartet with upright bass, guitar, etc.) on seemingly random evenings in the main restaurant - other nights nothing. There typically aren't any 'activities' at the beach area - although there is drink service.
I would highly recommend staying there if you want to vegetate, unwind, de-stess, read quietly, watch slow-moving boats, etc. yet still remain within easy reach of the golden zone and touristy areas, but if you want to be entertained and be active, I would recommend somewhere else.