Please get out the bikini/swimming costume you are planning to wear and try it on.
Just back from holiday and was surprised at what some women were wearing.
This isn't about how fat/thin they were just that the bikinis/costumes were often so so so unflattering and unsupportive.
My observations were that black/navy/chocolate brown looked very chic and flattered most women.
Lurid colours/patterns not such a good look. And if you can see the white threads of the lycra on your swimming costume you do really need to buy a new one.
HTH