I pretty much agree with everything I've read here. There has to a catch to the surprise excursion, and I think that catch is that the openers of the envelopes (Scott and Amy) will have to choose one person to be eliminated. Hence, Amy not wanting to choose one of her "friends" but having to since none of her enemies were on the cruise.
So Amy and Scott would have to choose one person, and since Scott wants to keep Holly, Des is gone. Adios, Amy's little lapdog.
The other side would be the Outsiders who weren't on the boat trip picking another person to be eliminated, which would easily leave Melanie odd girl out.
Props to whoever came up with this theory first. I think it's the most realistic scenario I've read.