Curly hair problems - Very tight and thin

I have really tight, pretty thin, curly hair. It is maybe a centimeter above my shoulders when it goes natural. I straighten it but when i jump in the pool, it goes frizzy and just looks terrible and i don't know what to do with it. I don't want to chemically straighten it because that would kill my hair. I don't know about getting a Japanese straight perm. I don't know if it will work on thin hair. I need ideas!

No Japanese straightening

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Don't ever get Japanese straightening. It will damage your hair beyond repair and your frizz will be double. I did it once and still regret it.