Blanche Tickle Girl by Wes Boyd
She doesn't stow away, but is invited. It is spur of the moment, though, and what develops from that is not foreseen by either of them.
It's the second of a series of four [at this point, anyway, I don't know if there is another in the eight books Wes had prepared prior to his death], but you don't need to have read the previous one, as the MC of this work only shows up in the last chapter of that volume, and what occurred then is covered at the very beginning of this volume.
And Australia is not involved. They do sail from Eastern Canada to the Mediterranean via Denmark, and then back, so lots of sailing. And a very nice romance.
The ending is a tearjerker, as boydpercy warned, but not unexpected; none of this walking across the street hit by a bus garbage, it's legit.
And there are loose threads as a result, which are dealt with in the following novels, so be warned, if you do decide to read it, and enjoy it, there are two more after it, and then you might want to read the first in the series, and after that, why, Wes wrote a bunch of books; 51 already published, the 52nd currently being posted, and eight more in the pipeline after that.
If you find you like this one, you could be set for reading material for quite some time...