The Secret Beach is located in the lower left corner of the Dark Portal (activate map to find Dark Portal). The arrow leading to the zone is invisible (click under your legs). It has the same normal monsters as The Beach (except Bonefish), with two boss monsters added (Water Spirit and Zombie Duck).
It is recommended to use heal against the Zombie Duck.
The Secret Beach is best farmed on casual difficulty due to the Zombie Duck's tenacity (Undead Boss), which means the normal Beach zone can provide comparable or better rewards in less time.