I successfully purchased free circulation (all-day pass) tickets for the Lake Como ferries for a weekday in mid-July! It is important to use a laptop and to buy the tickets well in advance of your trip. I couldn't purchase tickets with my smartphone. Step 1: Use this link for the official seller of Lake Como ferry tickets :
https://www.navigazionelaghi.it/en/search-results/
Step 2: Plug in the date and time you want the ticket for and choose your place of departure such as Bellagio and a destination such as Varenna and then push the "Search and Buy" button.
Step 3: On the right of your screen scroll down to the bottom. You will see two options: round trip and free circulation. Select free circulation. You won't be able to purchase more than 2 adult tickets at once. If you need more tickets, you will need to go through the process of buying the tickets again.
The tickets will be emailed to you as a PDF.
It might be more challenging to buy online tickets for the weekend.