The Surf Cabana is a hut in Super Mario Sunshine, found on Gelato Beach. It's notable for being placed on the water, and can be accessed by a boardwalk. According to the sign near the boardwalk and the owner of Surf Cabana, it's main feature is making juice smoothies from using various fruit and putting it into the juicer on top of the hut. Behind the hut is a little boardwalk with tables and umbrellas set up.
Despite appearing in every episode, not much attention is paid to it until Episode 8, where the Watermelon Festival is happening. During the watermelon contest, various Piantas and Nokis would try to present the biggest watermelon, and the biggest watermelon would be used to make juice smoothies, and the owner of the biggest watermelon would be rewarded with a Shine Sprite. Mario must present the biggest Watermelon there while avoiding Cataquacks. Once Mario does, the watermelon would be sliced on top of the juicer, and the Shine Sprite found locked in the cage would be presented to him.
Also worth noting is that Mario can throw in any other fruit in the juicer. He can grab the various fruits from on the beach or from the Sand Cabana. Once he throws a fruit in the juicer, a Blue Coin will pop out.