The Water Taxi runs a hop-on, hop-off service along the New River and the Intracoastal Waterway, stopping at a dozen or so points including Las Olas, the beach, the cruise port, and the strip of canal-front mansions known as Millionaires' Row. A day pass lets you ride as many times as you like, and the boats run frequently enough that you will never wait long. Buy tickets at any of the stops or directly on board.
The best strategy is to board at the Las Olas stop and head west first to get the downtown and river scenery, then swing east and north along the Intracoastal for the full mansion tour. At sunset the light on the waterways is genuinely good, and there are worse ways to spend an hour than watching egrets land on the docks while the boat idles past someone's private yacht. The Water Taxi also runs a dedicated Millionaires' Row tour if you want the narrated version — the guides are reliably entertaining on the subject of who owns what and for how much.