Cote, I am really sorry you have to endure what is now a farce of a boat trip. You are a steely woman, you will get through this.
I see the actual act of the affair as being quite irrelevant. Therefore, stopping it in its tracks before your dh goes on his trip does not achieve much, if I am thinking along the same lines as you.
You expect to be able to trust him NOT to do this. You will not be policing him or his fb account for the rest of your life. If a relationship comes to that, then query is it the right relationship?
Hence, you are more concerned with determining whether you can in fact Trust him (ie let him go on the trip under the watch of your PI, to flush out any lies), than with stopping any actual sexual infidelity. Not that the latter is not important, but more important is to determine how far the breakdown of trust is.
The uncertainty and doubt is far worse than having an answer, either way. I hope the answer will be the right one for you.