Sirena (mermaid) is in Parque Nacional Corcovado in Costa Rica. It is located on the tip of the Osa peninsula and at the far end of the park. It is highly suggested that you visit La Sirena if you want to fully appreciate Corcovado park.
Do bring alcohol or a soccer ball for the park guides. They don't get out much.
If you are planning on staying at La Sirena, please make a reservation with the park office in Puerto Jimenez because only a certain number of tourists are allowed in the park at a time. Dorms are provided at La Sirena, but you need to bring your own sheets. It is also permitted to camp at the station if you have your own tent. Bug nets are suggested. You can either bring your own food into the camp or dine at La Sirena with reservations.