For a bikini top with enough support and structure to fit a large bust properly, I'd recommend a tankini, halterkini or underwire bra bikini top.
The tankini is a swim top structured like a tank shirt, usually hip-length and lined with a shelf bra. Some also have extra shaping features, like tummy control and/or bust-minimizing bra cups. Tankinis are a flattering choice for most full-busted women.
A variation of the tankini is the halterkini, which has a single halter strap behind the neck instead of two straps over the shoulders. You might prefer a halterkini if you want to show a little more skin, as it leaves the shoulders and upper back bare.
You could also go with an underwire bikini top designed especially for full-figured women; these are sized like bras so you can be sure to get the right fit. They're available in standard cup sizes C through DD, and full-figure sizes E through L.
A few good brands for full-figured swimwear are Freya, Panache and Fantasie.