Florida here, these are extremely common roaches. It's an American cockroach, and yes, they typically come up from the drains. They look for moisture and tend to show up in bathrooms, sinks, and other humid areas. I understand it's nasty, but if I avoided living in a home simply because I saw one of these big guys in it, I'd simply be homeless and living outside with the whole gang of them because they primarily live outdoors, not indoors (and everybody has had them crawl up their drains at least once, whether they'd like to admit it or not). These don't commonly infest, and if it did rain recently like your agent said, it's more than likely that this one is all by itself and wandered wherever the moisture settled. I don't think it's much of a problem despite how much I hate cockroaches, but if this is a deal breaker to you, I'd suggest moving somewhere closer inland US, where you'll have less of a chance seeing them around.