We live near the beach so this was something I had to come to terms with pretty quickly. I'm a 50 year old flatchested beanppole; it's topless with board shorts in guy mode. But Sisterself is a bit more modest. Last year I wore a cute Jantzen one-piece that looked like a tankini- only had to wear that once to see how big a pain they are to take off to use the restroom once they're wet
I vowed to go two-piece this year. I'm only a 36A, got a cute Victoria Secret pushup halter bikini top and wore it with short shorts in the beginning of the season; short shorts are great if you are going to be more active. A bikini was a short leap from there so I picked up a cute matching bottom. Big improvement
over a one-piece. I basically wore bikinis the rest of the summer. I'm still a fan of tankinis for early in the season when I'm getting my base tan so I don't get any embarrassing lines for when I go topless presenting as a guy. LOL, I ended up cutting the suit I wore last year apart to make it into a real tankini!
I am bigender so I am pretty used to tucking; I don't fiddle with tape and just wear a Spanx "Slim Cognito" low rise panty whenever I present as female to hold things in place. A modest coverage bikini bottom hides the Spanx just fine if you roll down the waist. If it does happen to peak around the edges, it just looks like a beige liner.