Not going to get a straight answer from me. When I'm rowing I wear spash pants or nothing over my wetsuit, depending on just how cold it is. But as a paddler, either capitan or crew, I wear cut-offs. I find that my ass slips around on the tubes less with the added traction of the jeans material.
Butt for the fashion conscious it seems pretty obvious that the shorts should complement the paddling jacket and/or PFD.
But really, how badly do you want to win this argument? If I had a woman who wanted to raft with me often I'd let her wear what ever she wanted, even a wonderwoman, supergirl, ar catwoman outfit, and be proud to be seen with her.