There are many options for women to wear summer clothes and still carry concealed. Most of you know I encourage you to carry in the same way all, or most, of the time to avoid the moment of panic when you reach for your gun and it isn't where you trained for. But...there are times when that isn't possible. If you can't carry your normal holster, consider something like a Remora holster tucked into the general area where you usually carry. It doesn't take a lot of pressure on it to keep in place.
Flashbang makes options beyond the one you thing of, hanging from the center of the bra, although that can work if you are wearing a blousy peasant top. They also have one, for the LCP sizes gun, that tucks on the side of the bra. There are similar tuck into the waistband or boot models. for the bigger guns.
Best trick if lose of snug. You can still wear the snug tank top tucked into the cute little shorts to show of you summer shape, but you need a blouse or vest over the top. Aren't you glad retro crochet is in style? They make great cover garments.
In your bikini...not too many options. I don't advocate purse carry without a purse, but ...you could consider a pocket holster, or Remora, tucked into your beach bag. The thing to remember, is you can not leave your gun unattended.