Caja de Muertos is an uninhabited island nature reserve about eight miles off the southern coast of Ponce, accessible by boat from La Guancha. The island protects some of the most intact coral reef in Puerto Rico, and the snorkelling is exceptional: nurse sharks rest on the sandy bottom, hawksbill sea turtles cruise through the reef, and the water clarity makes every dive vivid. The beaches are white sand and largely empty outside of day trips, and the island's interior has hiking trails through dry tropical forest.
Several operators run half-day trips from La Guancha, typically departing in the morning and returning by early afternoon. The crossing takes about 45 minutes each way. The island is a nature reserve and camping is technically possible with permits, though most visitors do it as a day trip. Bring your own snorkel equipment if possible — rental is available but basic. It is the most rewarding single half-day excursion from Ponce.