I think it would be pretty bold to assume that it would be possible to migrate the other way. After all, they did make quite a dramatic historical point of the people moving to the dry land to seek their fortune and losing the ability to live in the sea. That's also the general atmosphere, and one reason why the sea folks are so careful not to dry too much: Because there's apparently no way to recover after the utmost limit.

But yeah, otherwise it would be quite cool if Tsumugu could sink to the bottom instead of Manaka becoming a landlubber.