I did NOT expect that. She was presented as a princess having a fetish for strong fighters, then as a diplomatic envoy, but now she is shown as having her own ambitions, clearly capable of handling herself if she can swim from one boat to another, and to be willing to make ridiculously bold moves to achieve them. I mean, she has admitted she has split away from her guards without revealing anything on where she intended to go, she is all alone, no allies, on a boat in the middle of the sea and those she is inconveniencing is a powerful merchant, a demon, and a party of strong adventurers. Did she not consider that it would be incredibly easy to dispose of her in this situation? How will the Empire ever find out what happened to her when the merchant's own subordinates are convinced she boarded her ship to sail back to the Empire? She's fully aware that the merchant is a man who is willing to use underhanded tactics so I guess she must have believed the Hero's party to be fairly naive and unwilling to allow killing her. Which is not the correct judgment of the MC, since the latter is fully willing to use an assassin to get rid of humans who he believes are a nuisance. It is true that the Princess presented a deal to the Merchant that he shouldn't pass up; after all she is asking for his support to her personally, in exchange for guaranteeing that the Empire won't take over the Merchant's realm. But did she really board that ship with that intention or was it a plan she decided on on the fly when she saw the demon? After all, if the merchant associates with demons, it explains why he doesn't feel threatened by the empire's open ambition to take over and it means that she hardly loses anything by proposing a personal alliance where she guarantees the continued independence of the merchants against their support. And she has to propose a deal to ensure her safety since she's gained dangerous knowledge concerning the merchant. Really, if she wanted to make that kind of deal, she could have done it before when they were talking so this deal probably is a result of her reassessing the situation when she saw that the merchant associated with demons.
Seriously, what was she thinking when she went and boarded herself to learn of what the merchant and hero were up to? Is she just that curious? Did she not consider the risks? There's no reason for the merchant to not plot her demise now that she thinks he has accepted her deal. What is she even going to report to her brothers when she goes back after having disappeared for the duration of this trip? The hero is right, she likely had another motive in boarding that ship, but I can't begin to guess any reason to risk her life for. It's all too foolish and lacks any sense of prudence.
@Syanalassa
We've seen one demon who is not affiliated with the Demon Lord who was trying to achieve something with the dragons. It's not just the human side who has shady characters. It's the combination of all those characters' underground actions that caused a world war.