I think she might be a Selkie! She has been going out with her friend to one of the local Lochs for some wild swimming and it seems to have made her immune to the cold waters!
I have swam in fairly cold waters when the air is in the twenties but the high teens just wont do!
Bare minimum I'll go into any kind of water is high 20s x_x Anything approaching 20 is too cold for me aaaaahhhhhhh.
I think that is a very sensible approach!