Sirena Beach

Sirena Beach is a part of Isle Delfino, a beach with warm, golden sunsets, target of electric Goop in Super Mario Sunshine, and the home of Hotel Delfino, a unique four-star hotel. It includes a snack bar, 2 pools (one is a sauna), and a Casino. The hotel is, in quite a few episodes of the level, being attacked by Boos. The hotel owner allows Mario to enter to hotel at no cost in return for exterminating the Boos and defeating Phantamanta.

Sirena Beach is also the level in which the Phantamanta boss is fought. You also fight King Boo here in the Casino. Peach, Toadsworth, and the Toads are probably meant to stay here, but because of the crisis it was cancelled. Once Mario enters the Casino he can't go back into the main lobby. And he also can't get back outside once he goes in the hotel. The pipe that leads to Sirena Beach is covered by a pineapple that only Yoshi can eat.

Trivia

 * Sirena is "Mermaid" in italian so the translation would be "Mermaid Beach".