Cheap and fun!
This has to be the cheapest and most fun type of marine transport I've ever taken in my life. They aren't just tourist boats, they're also used by locals to travel to work, especially at peak times, when traffic jams block the bridges. The trip takes about 15-20 minutes and costs about € 0.80 each way. Once you get to far side, the views of Lisbon are beautiful, with the seven hills, the San Jorge Castle, Commerce Square ... definitely worth it!