I don't think it's Itsuki because of the person watching them. I don't think it's Nino because she seemed surprised he was wet. I don't think it's Ichika for some reason. I've given thought to it being the mother, but that'd be weird time-wise(though it could be her this time and all 5 when they were younger). For some reason, I'm leaning towards Yotsuba because I'm pretty sure he'll end up with Yotsuba. She hides what she wants because what she wants is him, so he'll get her to focus on herself and, thus, they get married. I don't like that ending, but I'm betting on it.
Still, anyone but Ichika is still technically possible. Ichika won't win because I want her to.