Tri-Rail provides a useful rail link between West Palm Beach and Fort Lauderdale (plus south to Miami). It stops at Fort Lauderdale-Hollywood International Airport at Dania Beach. Although it's primarily for commuters, it can serve a useful means of getting around; you normally have to take bus connections from the station.
For the Treasure Coast, it's a bit trickier. A bus called the Treasure Coast Connector covers St Lucie County (Jensen Beach to Fort Pierce) though the sprawling nature of the suburbs can make it tricky to get around in a timely manner. While it's not impossible to get around on public transport, a vehicle is useful.