Guánica is a coastal town on the southern coast of Puerto Rico.
You pretty much need your own wheels to explore the area around Guánica.
The main reason to visit Guanica is the beaches. Take Route 333 south from the town to reach them. Caña Gorda Beach is the only one with facilities, but it's only open Tue-Sun.
From San Jacinto, just past Copamarina, you can also hop on a ferry excursion to Gilligan's Island (aka Cayo Caña Gorda), an undeveloped piece of paradise with white sandy beaches, snorkeling and mangroves.