It's another one of those subtropical days. You're hot, hot, hot. Your kids want out. Don't fall apart. Head for Castaway Island, where $5 will buy you or anyone else who's more than one year old admission to the coolest place in South Florida. (Babies get in free.) Opened in 1998 at a cost of $1.5 million, the island offers six slides, a water cannon, H2O-fed palm trees that will drench you, and a baby pool with even more amusements. Last year 140,000 people visited the playground, which is located in the middle of TY Park just west of I-95 and Sheridan Road. It's closed from November through mid-February but wonderfully wet the rest of the year. During the summer, when crowds get serious, the place is open from 8:30 a.m. to 6:30 p.m., and authorities limit you to two-hour sessions. But don't fret if you want to stay longer. There's an artificial lake and beach where you can cool your heels while you wait to reenter. And hey, there's one of those old-fashioned playgrounds with jungle gyms and slides just a few hundred yards away.