Thanks to demand by Christian women, you can now find truly modest swimwear. Below are some links to help you with the hunt, along with some notes on what I found as I searched.
Modestly Yours is almost all swim dresses with attached knee or capri length bottoms. Be sure to read the "About Us" page. You will find yourself wanting to buy from this site.
ModLi features swim tops with sleeves, swim bottoms that approach the knee and are covered by an opaque short skirt around the hips, as well as swim dresses with attached bottoms.
Undercover features swim dresses, full coverage tops, dark tees, and sarongs. They have both women and girls styles. In fact, several of these sites have both.
Caribbean Splash has primarily knee length swim dresses with attached bottoms. They feature bright colors and patterns, and have matching mom and daughter outfits which would be helpful for the girl who doesn't want to feel "weird."
HydroChic has swim dresses, full coverage tops, swim skirts, and both knee and capri length bottoms.
Primarily tops and knee length bottoms with a skirt around the hip area. As of the time I originally wrote this, the site was "on vacation" so check it every so often.
There are others listed. Just google "Christian swimwear." Oh, how I wish I had this when I was young. Look at every one of these and you will find something cute, I guarantee it. Other people will even think they are cute. But more than that, they will understand that there is something different about you—you are a godly woman and don't care who knows it.
But let your adorning be the hidden person of the heart with the imperishable beauty of a gentle and quiet spirit, which in God's sight is very precious. (1Pet 3:4).