Definitely wear what you feel comfortable in, most people won’t notice, and whatever they think is of no consequence.

I wear a one piece these days for regular swimming- it’s because I did have an unfortunate bikini top popping incident in the pool once so I feel more secure in my speedo one piece which is streamlined for swimming.

I would maybe wear a bikini if I was sunbathing on a hot beach though, not that it’s likely I will. My stomach isn’t flat btw, although I’m in healthy bmi.