First off, hair for me is always a problem because I have thick curly long hair and it gets really knotted when I go swimming. I've tried the signature topknot but it never seems to stay on top of my head and instead slowly falls out of position to the nape of my neck. Those of you who can pull that off I am eternally jealous. However, last time I went to the pool I tried braiding my hair 2 hours before hand in a fairly loose single braid and went into the water like that. After I came out and dried off a bit i took out the braid and my hair was what you always want it to look like at the pool at least for awhile. NO frizz, very wavy, and a little bit damp but not dripping so you long haired gals try that next time.
Next off makeup. I hope most people know less is more in almost all scenarios but especially when you go swimming. If you have a godsend of a waterproof mascara that you can't live without then go ahead but if you can help it avoid the full face of foundation, contouring (which is still confusing to me), and anything else you might put on your face.
Another important part in style is always accessories. Sunglasses are the easiest form of accessory to use in summer because they are also necessary. In the picture above I wear my Michael Kors sunglasses and in the picture below I'm wearing a gold cuff from Forever 21. (P.s. check out Aimee Song's book Capture Your Style it is the best read to learn more about Instagram but also has so many beautiful pictures inside.)
Finally clothes, while I always bring clothes for after, an extra shirt, things to wear under your clothes ;), I find that I enjoy wearing a pear of cutoffs and especially when you wear a one piece you can use that as your top temporarily (shirts like that actually exist anyways!).
I hope this helped all of you so enjoy your local swimming pools, water areas, or vacations and find what works for you!