In
el Puerto de Santa Maria the beach at the foot of
Avda. Bajamar seems to be more family-oriented (i.e., nothing risqué, no 'topless') but almost all beaches in Spain are now 'dress optional.' I'm sure there are other beaches nearby that are mostly 'topless.' You may have to take several types of bathing attire in order to 'fit in' everywhere.
There are probably days in October that might be considered 'beach weather,' but I think beaches in Alicante will be officially 'closed' by then. That just means there are no lifeguards on duty at
San Juan Playa, Playa Muchavista, Campello and even Benidorm. That doesn't seem to stop the tourists from northern Europe (who, by the way, are more likely to wear less than a full bathing costume).