@BRC,
Bottom line is that the two or you are not married. She has every right to do what she wants to do. Of course, that may carry consequences for her, such as damaging her relationship with you to the point that you decide to end it. She is obviously willing to accept that. So the ball is in your court. If the two of you were married (or even engaged) then I would agree that this boating trip was inappropriate.
The man she went boating with is an ass. He is the one obviously looking to cheat on his wife (assuming his wife was unaware of this one on one boating excursion.) But that is of course between he and his wife.
Basically, if what she did is unacceptable to you based upon where you think your relationship is or is headed, then I think you should end it. As for me, once my wife and my relationship got serious (back when we were dating many years ago) I would most certainly have had serious issues with her doing what your girlfriend did. And it would have nothing to do with my trusting her but rather everything to do with her not caring about how her actions affected me.