As I re-read the challenge I realized that a Survivor challenge on it's own wouldn't work (unless it's a case of the ship going down accidentally but that would mean that other people are also shipwrecked with them limiting the "Ordinary People" moments they could have).
Well, you could work it out in a couple of ways:
A) they could get shipwrecked while on an individual challenge (i.e. each couple to their own boat), but that would mean they'd have their cameraman with them, too. (What a case of Jimmy-interruptus!) The cameraman could get injured during the crash, leaving Clark with major guilt issues. Also, they would know that the show's producers would be out looking for them, unless -- of course -- that's who they were investigating in the first place for signing up with the show. Kind of a like a Noah's Ark meets Spencer's Island type thing.
B) Them being shipwrecked is part of the show, only the producers didn't tell them that was what the challenge was: Surviving a shipwreck. With each couple on the same island only several weeks apart. (Then they edit all the couples' adventures together to make it seem that they were all shipwrecked at the same time to different parts of the same island.) Clark realizes that there are cameras all over the island to film them for the show, so he can't do any super stuff while there to help them.