Just across the Golfo de San Miguel on Isla El Encanto are the ruins of the Fort of St Lawrence slowing being consumed by the jungle. There's not much more here than a 10m-tall watchtower with arrow slits, but it once played an important role in safeguarding the gold mines at Cana to the south.
You'll almost certainly see dolphins frolicking in the bay from the boat (US$15) as you cross.