If I was Finnish it would take a lot to make me want to put a ferry trip between myself and my departure airport (and as it would have to be a cost conscious customer then it would likely be a ferry trip), particularly just in terms of reliability and during the winter. I think the sad fact is that there are more Estonians willing to travel to Riga for flights than there are Finns willing to travel to Tallinn. (And indeed, there are probably more Estonians willing to travel to Helsinki for flights than vice versa).
Well, actually the ferry business is booming here. Even in Winter between Helsinki and Tallinn we have at least 13 connections each way (taking into account only vessels with min. 2000 pax capacity). The tickets are very cheap, so that many take a short trip every second week or so just to make their shopping on the other side of the Bay of Finland. Why not arrive by plane to TLL, buy a few cases of cheap beer and return by ferry home
?
However, I should mention also a newly refurbished low cost terminal in Tampere, just abandoned by Ryanair, standing totally empty right now. They are very eager to get ANY carrier to fly there. What about WAW - TMP to begin with?