Who cares? If you want to wear a bikini, wear a bikini.
Have you been to the beach lately? Nobody else seems to care how they look, so why should you? I’ve seen old guys with moobs in Speedos, overweight women in bikinis, men with huge beer guts, people leaving far too little to the imagination, and so on. And you know what? Good for them. They wear what makes them happy. If it grosses anyone out, who cares? Not them.
At any beach or pool you’ll always see more unfit people than fit. Most just look like normal people to me. I don’t think too many of us sit around thinking, “Oh, she really shouldn’t be wearing that.” Even if they are, what do you care? Do you know these people? Will you ever see them again? Nope. So f**k ‘em!
There are no set rules or guidelines. Wear what makes you comfortable. If you feel like people are staring and it makes you uneasy, then wear a one-piece or tankini or burlap bag. If you want to wear a bikini, do it, and proudly. You’re not out there for them, you’re out there for you. To have fun. You won’t have fun as long as you care so much what other people think.