bytheway My daughter is your height and weighs several stone more than you, and swimming including wild swimming is her passion. She swims several miles, several times a week and wears the usual speedo type swimming costumes that all regular swimmers well.
If you asked her, she would say what Callistemon says, once you get to your destination, you will meet people of all shapes and sizes in all kinds of swim suits and no one will take any notice of anyone else - unless of course you act all embarressed.
In the early years of GN, I won a Mother and Daughter Spa visit. DD and I went, but with some trepidation, as I too was several stone overweight at the time and we expected to find the spa full of elegant 'ladies who lunch' and beach babes.
When we got there we found lots of other mothers and daughters shaped just like us, and we had a whale of a time - and still are regular spa visitors, even DH comes and loves it - and he is relatively a lot more overweight than you are
So head up high, buy a costume you like, regardless of style and just get into it and enjoy yourself.