well technically, doesn't everyone swim with people in their underwear, that's basically what a swimsuit is.
But umm, this is a pretty touchy subject, because you could mention something about it and him not even look at her the way you're thinking, and you mentioning it will probably make him start putting it into perpective.
or if it turns out to be more than what it should be, then he'll obviously deny.
I've never been very much of the jealous type, so I probably wouldn't make a big deal. but that's just me.
try to ask another one of his friends about the new girl.