I have a Sea Folly tankini with underwire but I'm not sure whether they go up to F/G. I was an F pre-preg and wore my tankini anyway, it was an E but still fit fine. (I still squish into it now as I haven't been home to buy a new one yet!)
Haven't looked for togs on there yet but a friend of mine recommended zodee.com.au for a good range of "big" bras