From what I saw in the water scene, her dress isn't layered (besides her chemise underneath) in a way where it can be cleanly ripped apart. There's also a risk in just ruining the fabric / making the garment unwearable (bc u have no chance at threading them back tgt).
also this isn't as fair of a point, but she's still an esteemed noble of the empire, so running around in a thin and short dress (thin as in, since dresses in this time are made up of multiple layers, each layer isn't meant to keep one's body warm / isn't comfortable) is not really good for her reputation. She even made that comment to the prince regarding that bc he saw her body, she won't be able to marry. Even if they are on an island, she knows for a fact that they can return
We're on 2 different trains of thoughts. I answered on the premise of OP's original statement based on what I would do on a remote island with a historical dress, and not whether fMC actually could or couldn't.
Based on my knowledge of historical dresses, they are layered, and they are sewn in particular ways, and with even your nails or a thin enough pick, yes you can undo seams if you know where the seams are and their structure. Secondly even if they are thin layers, air between layers acts as an insulator. What keeps them cooler is not having physical contact with the dress, ie with a cage or petticoat, and a puffy dress like hers usually would have one to hold up the shape (regardless if she actually has anything under there, it looks to be at least 3 layers.) And if I were in her shoes, despite knowing I could go home, it wouldn't be for a year. At that point, I'd prioritize survival over customs and appearances because I personally know how cold it gets on an island.
So I don't want you to think I'm pushing this expectation upon the character, I just answered it in a way that's logical to my own knowledge and actions.
I see!! Thank you for clarifying that. I was definitely thinking more so in the realm of what our character could do. Also, I did not know that the air between the layers is what acts as an insulator. I guess where my concern lies is that, wouldn't the material /fabric of the layer impact how insulating it can get (even if they cover completely with shape)? For example, lightweight fabrics vs heavyweight fabrics?
Regardless, even if the layers were to be split apart, she would still be able to layer them on again at night to sleep for warmth so I guess my concern isn't that relevant lol

Imagine trying to survive on a remote island while wearing a big (possibly heavy) dress