Waze works in the US. The biggest issue will, as you say, be the high roaming charges. You can pre-load routes when you're connected to wifi to save some data, but then you won't get the real-time traffic updates obviously. I'd advise sorting out a data plan before you leave, either with your UK mobile provider or by using a US sim card when you're in Florida. Whatever you do, don't just leave your data on because it might run up a massive bill.