Thank you for the response. I hadn't looked at Viking Line. They have a deal where you leave Helsinki at 21:30 and can stay overnight on the boat, presumably at the Tallinn docks. 125 euros for two people in an outside cabin, or 190 euros for the Luxe cabins (probably not worth the extra). Quite economical since you save on a hotel. I'll definitely consider that. We are concerned about lots of drunks and noise on the overnight boat, but it will be on a Thursday.

No link for Viking Club on the English site, found it on Finnish site, but only residents of Finland are eligible.

The weather cancellations for the Linda Line are a worry, as we don't have a lot of flexibility in our itinerary.