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.
That's actually what I thought so too.
Also these relationships in real life always infuriated me. I always hated seeing my friends get into that kind of thing with their partners. Some of them knew they were in a abusive relationship and others had told me that at first they never realised it until it got much worse. A thing they both had in common was that they felt dependent on their partner. That there wasn't a reason to leave them.. So this manga is just one of those real life examples. Please if you know anyone in a similar situation just talk to them. Let them know that you and anyone else are there for them. That you care for their well being and that they are not alone.