Take the ATVO express bus from Marco Polo Airport to Venice Mestre train station: http://www.atvo.it/allegati/aeroporti/linea_35b_dal_29.10.2017_al_24.03.2018.pdf
I took it in the reverse direction (to the airport) last summer. It was quick (15-20 minutes), cheap (8 euro), and easy. From Venice Mestre station, you can get a fast train to Florence.
Quick comment: You may already know this, but if your departure flight from Venice leaves early in the morning, your transportation options from the city to the airport may be limited. People often start their trips in Venice instead of ending there for that reason.