Moshiach could come while people are in the mikvah, too. This is a silly thing for people to be getting worked up over,IMO. A swimming area is specifically for people to be wearing clothing meant for that area. As long as men are nowhere present, there is no justification for trying to make a case against swimwear that covers the essentials, even if it is not to your personal liking. then don’t wear it yourself. But please don’t think you are more proper than the lady who wants a better tan. And btw, I personally am on the conservative side when it comes to these things, and would never wear a two piece. But that is a personal preference, not because I feel they are untzniusdig in front of other women. If another woman likes the look, kol hakavod to her.
If there were a chance that any men might be able to see, then the regular bathing attire would ALSO be inappropriate, including the chassidic type (unless they cover the elbows and below the knees as well, and are not skin tight, of course).