She's actually wearing what's called a rash guard. They're not really a swimsuit, but they're not really a wetsuit either. Kind of in-between. Rash guards are tight fitting two-piece sportswear worn for extra function/protection when surfing, snorkeling, riding jet ski's, or doing other water sports where a swimsuit really isn't enough, but you don't really need a wetsuit either. Most rash guards have long sleeved tops, and while some show midriff, many have longer tops that partially cover the bottom piece (like the one Momiji is wearing here).
For tagging purposes though, it definitely fits the danbooru definition for swimsuit "A garment worn for swimming, bathing, and beach activities". Despite being a "protective" garment I think it still falls short of wetsuit "any protective bodysuit used in water activities such as surfing or scuba diving" since it's not actually a bodysuit (has a separate top and bottom).
I might look around later for more examples to see if it'd be worth making it's own subset tag under swimsuit. Not big on making a tag for just one picture.
