Oh but she WAS on the boat!
At least my brain is currently very convinced that Minna has confirmed that the baby Aino's expecting is Lalli, Tuuri and Onni's grandmother.
As for living on the boat, it's a possibility, but they do seem to have a land option because Tuuli mentions traditional sauna birth, and Eino speaks of a "rec room turned fallout shelter". Going by what would be really typical for Finns my theory is that they have a summer cottage + sauna on one of the gazillion little islands of the Saimaa lake system.
The boat can only be used for as long as there's gas for it, after which I assume they'll do the usual thing and use a small row boat instead (if you have a summer cottage next to a lake you most likely have a row boat). I guess they could try to re-stock on gas every once in a while, on those stations that are at smaller towns and villages... but that would be really risky and eventually those would run out of gas anyway.