Transtejo and Soflusa connect the two banks of the river with frequent and cheap rides.
The maximum price per person is Euro 2,40 on the longest ride. Group reductions apply.
From Belém depart ferry boats to Trafaria, via Porto Brandão; from Cais do Sodré depart ferry boats to Cacilhas, Seixal and Montijo; from Terreiro do Praço (near Praça do Comercio) you can find connections to Barreiro (the station with suburban trains headed to south Portugal).
Both companies, along with a handful of private companies, arrange short cruises of about 2h ½, with tourist guides in multiple languages.
The average price is about Euro 20 for adults and Euro 10 for children and aged people.