I believe that bringing her to the hot spring is a way for the husband to manipulate the wife into staying with him. He pretends to give her a present (the hot spring) as a way to have more emotional control in the relationship, to make the wife believe that he is capable of changing.
The fact that the hot spring water will cause even further pain to the wife's cuts symbolizes the husband's deceptive nature.

First time I read this one shot. Wish I hadn't.

The bastard taking his wife to a hot spring sounds like he's planning to murder her. Maybe I'm just watching too much detective drama. But still, I'd be very uncomfortable at the thought of an abuser taking his battered wife to a place where some spots would be secluded. I'd probably call the cops on him.