When I travelled from Portsmouth to Caen in March, the wifi in communal areas was free, there was a code printed on your boarding ticket. In cabins it was paid for only and not cheap. However... With the rules at the time, overnight you had to book a cabin and basically stay in it all the time, so you couldn't reach the free wifi. In the day if you booked a seat you could use it but had to wear a mask the whole time there, if you'd booked a cheap cabin you couldn't get to the free wifi except when you'd ventured out to the restaurant. Free wifi was pretty slow even with just a handful of passengers on the ferry.

I think they have relaxed some rules now, so it be easier to spend time in range of the free wifi, but don't expect much from it!