Whoever you spoke to was incorrect or misunderstood you.
The Federal park (Matanzas) is a HUGE NO NO.
You are allowed to detect Anastasia State park on the wet sand only.
Again the pier is within city limits but the no detecting law there is not enforced on the beach.
Happy hunting
You can the actual laws here:
https://www.municode.com/library/fl..._of_ordinances?nodeId=PTIICOOR_CH6ARPR_S6-3DE
Saint Augustine City -The pier is in the city limits
Sec. 6-7. - Excavations on public property.
No individual shall be allowed to use a probe, metal detector or any other device to search or excavate for artifacts on public property, nor can any individual remove artifacts from public property without the written permission of the city. Furthermore, no disturbances or construction activities shall be authorized within properties belonging to the city, including public streets and rights-of-way, without a city right-of-way permit and without such archaeology efforts as may be addressed by this chapter. Any proposed archaeological work and delays relative to a disturbance or construction work shall be in accordance with provisions of this chapter relative to major and minor disturbances in Archaeological Zones I, II and III.